Laboratoire d’Études du Rayonnement et de la Matière en Astrophysique et Atmosphères


Séminaires passés (Ordre chronologique inverse)

Vendredi 19 mars 2021, 14h00
Visioconférence via Zoom, VIDEO
Millimeter rotational lines as powerful diagnostics of the physical conditions inside a Giant Molecular Cloud - The Orion B case
Jérôme PETY
IRAM/Observatoire de Paris-LERMA
résumé :
Molecules have long been thought to be versatile tracers of cold neutral media in the universe, from high-redshift galaxies to star forming regions and proto-planetary disks, because their internal degrees of freedom are controlled by physical conditions in their environments. However, the promise that molecular emission has a strong predictive power of the physical and chemical state of the interstellar medium is still hampered by the incomplete understanding of the complex physical and chemical structure of the interstellar gas, and its dynamical evolution.

In this talk, I will present The ORION-B project (Outstanding Radio-Imaging of OrioN-B), a Large Program of the IRAM 30m telescope, and discuss three recent results obtained thanks to machine learning techniques:

1) How to improve the quantitative estimate of the H2 column density based on the molecular line intensities (Gratier et al. 2021);

2) How to estimate a reference precision on the column densities, excitation temperatures, centroid velocities and velocity dispersions of the three main CO isotopologues, 12CO, 13CO, C18O, with the help of the Cramer Rao Bound (CRB) technique (Roueff et al. 2021);

3) How to identify the best tracers of the ionization fraction among hundreds of species included in models (Bron et al. 2021). I will discuss the astrophysical consequences.


* Bron et al. 2021, A&A, 645, 28B, "Tracers of the ionization fraction in dense and translucent gas. I. Automated exploitation of massive astrochemical model grids"

* Gratier et al. 2021, A&A, 645, 27G, "Quantitative inference of the H2 column densities from 3 mm molecular emission: case study towards Orion B"

* Roueff et al. 2021, A&A, 645, 26R, "C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud. Analysis of the achievable precision in modeling spectral lines within the approximation of the local thermodynamic equilibrium"

Vendredi 5 mars 2021, 14h00
Visioconférence via Zoom, video
Laboratory astrophysics studies of VUV and X-ray induced photodesorption from interstellar ice analogues
Mathieu BERTIN
résumé :
The recent advances in space and ground based telescopes (ALMA, NOEMA…) have allowed the detection of more and more molecules in the gas phase in the coldest regions of the interstellar medium - ISM (star-forming regions, protoplanetary disks…). The puzzling detection of these gaseous species, including small organic molecules, in media where the temperature is very low (~ 10-100 K), is currently a major and still open question, directly linked to the astrochemical richness. Most of the observed molecules are indeed expected to either directly form or accrete on the surface of dust grains, and cannot thermally desorb in the regions where they are detected. Their observation requires thus non-thermal desorption processes, among which the desorption induced by UV or X-ray photons – so-called photodesorption – is a promising candidate. However, its role still needs to be clarified, especially in the case of the desorption of small organics molecules for which both the quantitative yields and the underlying mechanisms are lacking.

I will present the outcomes of recent laboratory astrophysics studies base on the use of the monochromatic and tunable synchrotron radiation, dedicated to understand and quantify the photodesorption processes in both the vacuum UV (7-13,6 eV) and soft X-rays (500 – 1500 eV) energy ranges. The role played by the photon energy and of the molecular ice composition on the desorption yields will be highlighted, and a special focus will be made on the case of photodesorption of complex organics molecules.

Vendredi 26 février 2021, 14h00
Téléconférence via Zoom, video
Morphological Transformations and Quenching of Galaxies : from Simulations to Observations
LERMA-Observatoire de Paris-PSL
résumé :
The origins of the relation between galaxy structure and star formation is still debated. I will discuss recent efforts to advance in our understanding of how massive galaxies change their morphology and quench from z~3. Using several state of the art deep learning techniques, we try to link hydrodynamic cosmological simulations with observations from deep surveys to constrain the physical conditions and evolutionary tracks of galaxies.

Vendredi 22 janvier 2021, 14h00
via Zoom , Paris
Dark matter halo response to baryons
Observatoire astronomique de Strasbourg
résumé :
While cold dark matter numerical simulations predict steep, `cuspy' density profiles for dark matter halos, observations favour shallower `cores'. The introduction of baryonic physics alleviates this discrepancy, notably as feedback-driven outflow episodes contribute to expand the dark matter distribution for stellar masses between 10^7 and 10^10 Msun. I will first present a parametrization of dark matter halo density profiles with variable inner slope and concentration that enables to describe the variety of halo responses to baryons and has analytic expressions for the gravitational potential, the velocity dispersion, and lensing properties. This parametrization provides a useful tool to study the evolution of dark matter haloes, to model rotation curves of galaxies and gravitational lenses, and to be implemented in semi-analytical models of galaxy evolution. I will then present two theoretical models describing core formation in dark matter haloes. In the first one, sudden bulk outflows induced by stellar feedback reorganise the halo mass distribution while it relaxes to a new equilibrium. In the second one, small stochastic density fluctuations induce kicks to collisionless particles that progressively deviate them from their orbits. Both models are tested against numerical simulations and provide a simple understanding of the transition from cusps to cores by feedback-driven outflows.
Vendredi 11 décembre 2020, 14h00
via Zoom,
Investigating the physical processes driving the evolution of baryons in local and high-redshift low-metallicity galaxies
Laboratoire d'Astrophysique de Marseille & National Centre of Nuclear Research, Warsaw
résumé :
The chemical enrichment in the interstellar medium of galaxies is regulated by several physical processes: stellar birth and death, dust growth and destruction, galactic inflows and outflows. Understanding the interplay of such processes is essential in order to study galaxy evolution, the chemical enrichment of the Universe through the cosmic epochs and to interpret the available and future observations. Despite the importance of such topics, the contribution of different stellar sources to the chemical enrichment of galaxies, e.g. massive stars exploding as Type II supernovae and low-mass stars, as well as the mechanisms driving the evolution of gas, metal and dust grains, remains controversial. In this seminar, I will revise our current knowledge on these physical processes and the observational challenges. I will then present the results of a recent investigation focused on local low-metallicity galaxies for which the evolution of metals, gas and dust content has been studied. In particular, I will show how the comparison between model predictions and observations can allow us to identify the most relevant physical processes determining the chemical evolution of these systems. I will then discuss how the information derived for local low-metallicity galaxies can be employed to study Lyman-Break Galaxies at the epoch of reionization, which are often considered to be their high-redshift counterparts.

Vendredi 4 décembre 2020, 14h00
via Zoom,
Lien : https://vm-weblerma.obspm.fr/~lestrade/2020_12_04_YohanDubois.mp4
Simulating galaxies at high resolution in their cosmological context with NewHorizon: methods and some key results on galaxy properties and their morphology
Institut d'Astrophysique de Paris
résumé :
Hydrodynamical cosmological simulations are increasing their level of realism by considering more physical processes, having more resolution or larger statistics. However, one usually has to either sacrifice the statistical power of such simulations or the resolution reach within galaxies. I will introduce the NewHorizon project where a zoom-in region of ~(16 Mpc)^3, larger than a standard zoom-in region around a single halo, embedded in a larger box is simulated at high resolution. A resolution of up to 34 pc, typical of individual zoom-in state-of-the-art resimulated halos is reached within galaxies, allowing the simulation to capture the multi-phase nature of the interstellar medium and the clumpy nature of the star formation process in galaxies. I will present and discuss several key fundamental properties of galaxies and of their black holes. Due to its exquisite spatial resolution, NewHorizon captures the inefficient process of star formation in galaxies, which evolve over time from being more turbulent, gas-rich and star-bursting at high redshift. These high redshift galaxies are also more compact, and are more elliptical, disturbed and clumpier until the level of internal gas turbulence decays enough to allow for the formation of stable rotating discs. I will show the origin and persistence of the thin and thick disc components, and explain why the settling of discs ``magically’’ occurs at around a stellar mass of 1e10 Msun.

Vendredi 27 novembre 2020, 14h00
via Zoom,
La phosphine sur Vénus : Une histoire brève qui en dit long
LESIA, Observatoire de Paris
résumé :
Le 14 septembre 2020, une nouvelle fait sensation dans le monde des astronomes… et au-delà. A partir de données obtenues avec le JCMT et ALMA, une équipe internationale pilotée par Jane Greaves (Université de Cardiff, UK) annonce la détection de la phosphine (PH3) dans l’atmosphère de Vénus et évoque la possibilité d’une forme de vie sur la planète. Plusieurs articles sont publiés ou soumis dans la foulée, étudiant les qualités éventuelles de la phosphine en tant que biomarqueur ou évoquant un développement possible de la vie dans les nuages de Vénus, et la presse internationale en fait grand cas. De notre côté, au LESIA, nous menons depuis huit ans une campagne d’observations de Vénus en infrarouge, avec le télescope IRTF de Maunakea, pour étudier les variations de deux molécules mineures importantes pour la climatologie de Vénus, SO2 et H2O. En mars 2020, nous avons été contactés par l’équipe de J. Greaves pour rechercher la phosphine dans l’infrarouge avec l’instrument que nous utilisons. Les observations n’auront pas lieu à cause de la crise sanitaire. Cependant, nous avons recherché la signature de PH3 dans d’anciennes données infrarouges prises avec le même instrument, et nous en avons déduit pour l’abondance de PH3 une limite supérieure très contraignante, quatre fois plus faible que la valeur annoncée par Jane Greaves et ses collègues. Par ailleurs, le traitement des données ALMA par son équipe fait l’objet de nombreuses réserves ou critiques de la part des radioastronomes. Il est donc vraisemblable que l’engouement des média pour la phosphine de Vénus va retomber prochainement…
Vendredi 20 novembre 2020, 14h00
via Zoom,
Recent results from scattering transforms in astrophysics and cosmology
ENS, Paris
résumé :
New statistical descriptions related to the so-called scattering transforms recently obtained attractive results for several astrophysical applications. These statistics share ideas with with convolutional neural networks, but do not require to be learned, and allow direct characterization of interactions between scales in non-linear processes. In this talk, I will introduce these statistical descriptions, and present the results obtained on interstellar turbulence (characterization, classification and denoising) as well as on the cosmological density field (inference of cosmological parameters and statistical syntheses).
Vendredi 23 octobre 2020, 14h00
téléconférence Zoom,
The role of molecular filaments in the origin of the IMF
Philippe ANDRÉ
CEA, Laboratoire d’Astrophysique AIM Paris-Saclay
résumé :
The origin of the stellar initial mass function (IMF) is one of the most debated
issues in astrophysics. I will discuss new insights into this problem based on a systematic census of prestellar cores and molecular filaments in nearby clouds taken as part of the Herschel Gould Belt survey, as well as higher-resolution observations with APEX/ArTéMiS and ALMA. Our results point to the key role of the quasi-universal filamentary structure pervading molecular clouds. They suggest that the dense cores making up the peak of the prestellar core mass function (CMF) - and indirectly the peak of the IMF - result from gravitational fragmentation of molecular filaments near the critical mass per unit length. The Salpeter power-law tail of the CMF/IMF may be at least partly inherited from the filament line mass function (FLMF), which is observed to follow a Salpeter-like power law in the regime of thermally supercritical filaments.

Vendredi 3 juillet 2020, 14h00
Metallicity gradients of star-forming regions since cosmic noon
Patricia B. TISSERA
Universidad Andrés Bello, Santiago
résumé :
The evolution of chemical abundances in galaxies provides crucial information to study how galaxies formed and evolved. Results in the Local Universe show that on average, disc galaxies have negative metallicity gradients. As a function of redshift, the trend is not clear enough yet, although there is a significant fraction of positive ('inverted') metallicity gradients.

Using hydrodynamical cosmological simulations, we studied the metallicity gradients at z=0 and the processes that might affect them and the azimuthal metallicity distribution such as mergers, SN and AGN feedback and gas inflows.

In this talk I am going to summarize the latest results on the evolution of the metallicity gradients as a function of stellar mass, galaxy size and star formation efficiency from z~0 to z~2.5.
Vendredi 12 juin 2020, 14h00
Observational and theoretical constraints on the formation and early evolution of the first dust grains in galaxies at 5 < z < 10
Laboratoire d'Astrophysique de Marseille
résumé :
The first generation of stars were born a few hundred million years after the big bang. These stars synthesized elements heavier than H and He, that are later expelled into the interstellar medium, initiating the rise of metals. Within this enriched medium, the first dust grains formed. This event is cosmological crucial for molecule formation as dust plays a major role by cooling low-metallicity star-forming clouds which can fragment to create lower mass stars. Collecting information on these first dust grains is difficult because of the negative alliance of large distances and low dust masses. We combine the observational information from galaxies at redshifts 5 < z < 10 to constrain their dust emission and theoretically understand the first evolutionary phases of the dust cycle. Spectral energy distributions (SEDs) are fitted with CIGALE and the physical parameters and their evolution are modelled. From this SED fitting, we build a dust emission template for this population of galaxies in the epoch of reionization. Our new models explain why some early galaxies are observed and others are not. We follow in time the formation of the first grains by supernovae later destroyed by other supernova blasts and expelled in the circumgalactic and intergalactic media.
Vendredi 29 mai 2020, 14h00
Salle de l'atelier, Paris
The Origins Space Telescope
Observatoire de Paris/LERMA
résumé :
The Origins Space Telescope is one of four large satellite studies thathas been submitted to the Astro2020 Decadal Review. In the talk I will briefly lay out the science that defined the design of Origins. The satellite and its 5 instruments will be introduced. The HEterodyne Receiver for Origins (HERO) is one of the 5 instruments designed for Origins. I will give an overview of the HERO design and its defining features. HERO is the first heterodyne array
receiver designed for a satellite.

Vendredi 15 mai 2020, 14h00
The challenges of observing the Epoch of Reionization and Cosmic Dawn
Kateyn institute
résumé :
Low-frequency observations of the redshifted 21cm line promise to open a new window onto the first billion years of cosmic history, allowing us to directly study the astrophysical processes occurring during the Epoch of Reionization (EoR) and the Cosmic Dawn (CD). This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under astrophysical foregrounds orders of magnitude brighter and contaminated by numerous instrumental systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are currently underway aiming at statistically detecting the 21-cm brightness temperature fluctuations from the EoR and CD. While no detection is yet in sight, considerable progress has been made recently. In this talk, I will review the many challenges faced by these difficult experiments and I will share the latest development of the LOFAR Epoch of Reionization and NenuFAR Cosmic Dawn key science projects.
Jeudi 30 avril 2020, 14h00
The Great Dimming of Betelgeuse
KU Leuven, Institute of Astronomy
résumé :
Red supergiant stars are important contributors to the chemical enrichment of the Universe. Together with asymptotic giant branch stars, their lower mass counterpart, they contribute ~85% of gas and ~35% of dust to the total enrichment of the interstellar medium. Moreover, the stellar wind has a crucial impact on the final mass, hence on the nature of the compact remnant left after the supernova: a 20 solar mass star can loose up to 60% of its mass during its life. Yet the mechanism at the origin of the red supergiant mass loss remains unknown: there is no physical scenario to lift material from the photosphere up to the dust condensation zone where radiative pressure on small grains can drive the wind.

In November-December 2019, the prototypical red supergiant Betelgeuse started an impressive dimming that brought it to 37% of its average optical brightness in February 2020. It is dimmer than this star has been since quantitative magnitude measurements have been recorded (150 years). We have observed Betelgeuse at high angular resolution during this peculiar event with the VLT/SPHERE, VLTI/GRAVITY and VLTI/MATISSE instruments. I will present the first results of this multi-wavelength and multi-technique campaign and bring them in the context of the study of the red supergiant mass loss.
Vendredi 17 avril 2020, 14h00
Salle de l'atelier, Paris
Patricia TISSERA
Universidad Andres Bello, Santiago, Chili
Vendredi 3 avril 2020, 14h00
Salle de l'atelier, Paris
The great dimming of Betelgeuse in 2019-2020
KU Leuven, Institute of Astronomy
résumé :
Red supergiant stars are important contributors to the chemical enrichment of the Universe. Together with asymptotic giant branch stars, their lower mass counterpart, they contribute ~ 85% of gas and ~ 35% of dust to the total enrichment of the interstellar medium. Moreover, the stellar wind has a crucial impact on the final mass, hence on the nature of the compact remnant left after the supernova: a 20 solar mass star can loose up to 60% of its mass during its life. Yet the mechanism at the origin of the red supergiant mass loss remains unknown: there is no physical scenario to lift material from the photosphere up to the dust condensation zone where radiative pressure on small grains can drive the wind.

In November-December 2019, the prototypical red supergiant Betelgeuse started an impressive dimming that brought it to 37% of its average optical brightness in February 2020. It is dimmer than this star has been since quantitative magnitude measurements have been recorded (150 years). We have observed Betelgeuse at high angular resolution during this peculiar event with the VLT/SPHERE, VLTI/GRAVITY and VLTI/MATISSE instruments. I will present the first results of this multi-wavelength and multi-technique campaign and bring them in the context of the study of the red supergiant mass loss.
Vendredi 20 mars 2020, 14h00
Salle de l'atelier, Paris
The challenges of observing the Epoch of Reionization and Cosmic Dawn
Kateyn institute
résumé :
Low-frequency observations of the redshifted 21cm line promise to open a new window onto the first billion years of cosmic history, allowing us to directly study the astrophysical processes occurring during the Epoch of Reionization (EoR) and the Cosmic Dawn (CD). This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under astrophysical foregrounds orders of magnitude brighter and contaminated by numerous instrumental systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are currently underway aiming at statistically detecting the 21-cm brightness temperature fluctuations from the EoR and CD. While no detection is yet in sight, considerable progress has been made recently. In this talk, I will review the many challenges faced by these difficult experiments and I will share the latest development of the LOFAR Epoch of Reionization and NenuFAR Cosmic Dawn key science projects.
Vendredi 28 février 2020, 14h00
Salle de l'atelier, Paris
Filament Paradigm and Galactic Star Formation
Shu-ichiro INUTSUKA
Nagoya University
résumé :
Recent observations have emphasized the importance of the formation and evolution of magnetized filamentary molecular clouds in the process of star formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds. Thus, the mass function and rotations of molecular cloud cores should be directly related to the properties of the filamentary molecular cloud, which determines the initial size and mass distribution of a protoplanetary disk around a protostar created in a core. In this talk I explain our current understanding of the star formation processes in the Galactic disk, and summarize various processes that are required in describing the filamentary molecular clouds to understand the star formation rate/efficiency, the stellar
initial mass function, and the angular momentum distribution of protoplanetary disks in their early evolutionary phase.
Vendredi 21 février 2020, 14h00
Salle de l'atelier, Paris
Angular momentum properties of young protostellar envelopes
Mathilde GAUDEL
résumé :
One of the main challenges to the formation of solar-like stars is the “angular momentum problem”: if the angular momentum of the pre-stellar parent core is totally transferred to the central stellar embryo during the main accretion phase, the gravitational force can not counteract the centrifugal force and the embryo fragments prematurely before reaching the main sequence. To form a star such as our Sun, the gas of the rotating envelope needs to redistribute its angular momentum by 5 to 10 orders of magnitude before reaching the central stellar embryo. Class 0 protostars are key objects to identify the mechanisms responsible for the angular momentum redistribution : they grow by accretion of the matter from the surrounded envelope (Menv>>Mstar) extending to scales 10000 au. At the end of this cornerstone phase, most of the final stellar mass has been accreted and the embryo is surrounding by a large disk (~100 au).

In order to tackle this issue, we used high angular resolution observations (0.5’’, i.e. ~50 au) from the CALYPSO (Continuum and Lines in Young Protostellar Objects, PI: Ph. André) IRAM large program for a sample of 12 Class 0 protostars with d<400 pc. We established, for the first time homogeneously in a large sample, robust constraints on the radial distributions of specific angular momentum within protostellar envelopes in a large range of scales from ~50 to 10000 au (Gaudel et al. 2020, submitted). Two distinct regimes are revealed: a constant profile at small scales (<1600 au) and an increasing of the angular momentum at larger radii (1600-10000 au).

From the constant profile, I will discuss angular momentum conservation and disk formation as possible solutions to reconnect the angular momentum measured in the inner protostellar envelopes to what is expected in T-Tauri disks. Furthemore, velocity gradients observed on large scales (>3000 au) - that are historically used to measure the rotation of the core and quantify the angular momentum problem - are not due to pure envelope rotation. I will examine the influence of the interstellar filament dynamics (turbulence, collapse, shocks) within which protostars are buried and the imprints of the initial conditions of the pre-stellar phase in the large scales of the envelope.
Vendredi 31 janvier 2020, 14h00
Salle de l'atelier, Paris
The role of feedback- and accretion-driven turbulence in galaxy build-up
résumé :
Cosmological models describe accurately the growth of large scale, dark matter-dominated, structures, but largely fail to reproduce the baryon content and physical properties of galaxies. Why? Essentially because the build-up of galaxies is regulated by a complex interplay between gravitational collapse, galaxy merging and feedback related to AGN and star formation, for which we still miss a robust theory. The energy released by these processes has to dissipate for gas to cool, condense, and form stars. How gas cools is thus a key to understand galaxy formation and why it such an inefficient process. In this seminar, I will discuss a few examples where turbulence driven by gas accretion, feedback, and galaxy interactions, which is largely ignored in models of galaxy formation, and captured in current simulations only over a limited range of scales, may have a major impact on galaxy and halos properties.

Vendredi 24 janvier 2020, 14h00
Salle de l'atelier, Paris
Unveiling the accretion-ejection connection in nascent Sun-like stars
résumé :
Disks around nascent stars are the birthplace of planets. However, many critical aspects of planet formation are still highly uncertain, in particular the impact of winds on disk evolution. Indeed, magnetic outflows launched from the disk surface (‘MHD disk-winds’) have recently been invoked to extract angular momentum and drive accretion, regulate dust growth, and prevent inward planet migration. The unique combination of sensitivity and angular resolution offered by ALMA and the future JWST is now allowing us to conduct stringent tests on the presence of MHD disk-winds and quantify their impact on planet-forming disks.

In this seminar, I first present an ALMA study of the Sun-like protostellar system HH212 that has uncovered a SO/SO2 rotating flow wider and slower than the axial rotating SiO jet. The global kinematics and the mass-flux of the flow down to 0.04-0.02” (16-8 au) scales may be fitted by an MHD disk-wind model launched out to the outer disk radius (~40 au), with SiO tracing the dust-free streamlines from 0.05?0.3 au (Tabone+2017; Tabone+2020a). The surprising molecular richness of the dust-free part of the wind is further investigated with astrochemical models (Tabone+2020b). The extension of such tests to more evolved sources, together with tests based on the observed perturbation of the wind by bow-shocks (Tabone+2018), are also discussed. Finally, I present my current modelling effort to derive the local UV radiation field in disk atmospheres - a key parameter that controls the chemistry of the disk and the launching of the wind - from the OH lines to be observed by JWST/MIRI.
Vendredi 13 décembre 2019, 14h00
Salle de l'atelier, Paris
Falsifying the concordance of cosmology with the large-scale structures
Yonsei University, Seoul
résumé :
Despite great predictive power and its successes in the last decades, the concordance LCDM cosmological model suffers from both observational and theoretical issues. On the theoretical side, while a dark energy component is needed to explain the late-time acceleration of the Universe, its nature is unknown, and the value of the cosmological constant has to be fine tuned. On the observational side, tensions between cosmic microwave background and late Universe measurements (H0, sigma8) are getting increasingly hard to alleviate.
Therefore, it is important to further test the model. Falsification is an important concept, and can possibly lead to paradigm changes. In this talk, I will focus on model-independent tests of different aspects of the concordance model such as the curvature, the metric, or gravity, in an attempt to falsify it using state-of-the-art cosmological data: type Ia supernovae, baryon acoustic oscillations, and redshift-space distortion. In particular, I will focus on model-independent methods such as iterative smoothing, Gaussian process regression, or crossing statistics, which are a powerful tool for model falsification. While having less constraining constraints than model-dependent approaches, they have more flexibility, are not biased towards a certain model, and can lead to the detection of unexpected features or reveal the presence of systematics in the data.
Mardi 3 décembre 2019, 14h30
Salle DANJON, Paris
Seminaire LERMA exceptionnel dans la salle DANJON
Charting The Next Frontier Of Nanohertz Gravitational Wave Astronomy With The International Pulsar Timing Array
Stephen TAYLOR
Vanderbilt University
résumé :
The supermassive black holes that lurk in galaxy centers should form binaries as galaxies merge over cosmic time, emanating gravitational waves in the nanohertz sensitivity band of networks of Milky Way millisecond pulsars. Pulsar-timing arrays (PTAs) are poised to chart this new frontier of gravitational wave discovery within the next several years. With this new window onto the warped Universe, PTAs will bring a dramatic change in our understanding of supermassive binary black-hole demographics and dynamical interactions. The International Pulsar Timing Array (IPTA) combines precision timing data from regional collaborations in Europe, North America, Australia (and new partners in South Africa, India, and China) to accelerate these goals. I will review ongoing work by my IPTA colleagues to place stringent gravitational-wave limits and increase near-future detection prospects using the recently published "IPTA Data Release 2".

Vendredi 29 novembre 2019, 14h00
Salle de l'atelier, Paris
The size of galaxies in the era of ultra-deep imaging
Nushkia CHAMBA
Instituto de Astrofisica de Canarias
résumé :
While the effective radius is a robust parameter, its use to
characterise galaxy sizes has provided a counter-intuitive definition of
what the actual extent of a galaxy is. Current deep imaging therefore
offers a unique opportunity to critically review the convention that the
size of a galaxy is its effective radius and rethink how one best
measures the extent of galaxies using a physically motivated parameter.
We introduce a new definition of galaxy size based on the gas density
threshold for star formation in galaxies. Remarkably, our new size
definition not only captures what the human visual system identifies as
the edge of a galaxy, but also dramatically decreases the scatter in the
stellar mass - size plane by a factor of three. Our size parameter
unifies galaxies spanning five orders of magnitude in stellar mass on a
single mass-size relationship. To demonstrate the implications of our
results, we show that ultra-diffuse galaxies have the same sizes as
regular dwarfs when a size indicator that describes the global structure
of galaxies is used. This work may be extended for larger samples of
galaxies using upcoming wide, deep imaging surveys.
Vendredi 15 novembre 2019, 14h00
Salle de l'atelier, Paris
Excitation mechanisms in the intracluster filaments around the Brightest Cluster Galaxies
Fiorella POLLES
résumé :
In the center of galaxy clusters lie giant elliptical galaxies, the Brightest Cluster galaxies (BCGs). These galaxies are the best environment to study the interaction between Active Galactic Nuclei (AGN) and Intra Cluster Medium (ICM) and to understand the life cycle of gas in presence of feedback. The BCGs are often surrounded by a system of filaments (e.g. Salom'e $&$ Combes 2003) that emit in a wide range of wavelengths, illustrating the multi-phase nature of these streams. Many of these filaments do not have strong on-going star formation and the photoionization by stellar emission cannot reproduce their emission (Johnstone et al. 2007): How are they form? what is preventing these structures to create stars?

We studied the ALMA and MUSE date of a sample of BCGs. The radial profile of the H$alpha$/CO flux ratio is roughly constant for most of the objects. This suggests that local processes are responsible for exciting the filamentary emission. The best BCG to investigate the heating source is the NGC1275, the central giant elliptical galaxy of the Perseus Cluster, which is the X-ray brightest cluster in the sky. We have investigated cosmic rays and X-rays as likely heating sources, combining multi-wavelength line emission (from optical to far-infrared) with models produced using the photoionization and photodissociation code Cloudy. We have fully constrained the model of the ionized phase and pushed the analysis to the molecular phase. We showed that using X-ray emission as the main heating source, all of the ionized line emission can be reproduced. We found that to reproduce [OI]63$mu$m line, a small filling factor of the photodissociation phase is necessary. Finally, we also showed that adding an additional dense phase or an extra pressure component is required to robustly reproduce the H2 line emission.

Vendredi 11 octobre 2019, 14h00
Salle de l'atelier, Paris
Galaxy clusters and proto-clusters at z>1.5
Simona MEI
résumé :
We will give a short overview of the field and present recent results on galaxy stellar population and morphology in clusters and proto-clusters detected as overdensities around radio-loud AGN at z>1.5 from the CARLA (Clusters Around Radio-Loud AGN) survey. Galaxy clusters in the local Universe predominantly host a population of massive, passive early-type galaxies, which formed the bulk of their stellar populations at redshifts z~2-3. The progenitors of the present-day clusters are clusters and proto-clusters at z>1.5 that are still in the process of assembling. CARLA is the largest homogeneous sample of spectroscopically confirmed clusters and proto-clusters currently available, with 16 clusters and proto-clusters that cover the redshift range 1.6<z<2.8. At these redshifts, we can observe changes in the cluster and proto-cluster galaxy populations both in morphology and star formation quenching. Our results are based on observations with HST/WFC3 and observations of the cold gas around the AGN and in surrounding galaxies with IRAM/NOEMA, ALMA and Keck KWCI. We will compare our sample to samples observed at the same wavelengths and obtained with other selection criteria.
Vendredi 20 septembre 2019, 14h00
Atelier, Paris
Challenging a Newtonian prediction through Gaia wide binaries
UNAM, Mexico
résumé :
Under Newtonian dynamics, the relative motion of the components of a binary star should follow a Keplerian scaling with separation. Once orientation effects and a distribution of ellipticities are accounted for, dynamical evolution can be modelled to include the effects of Galactic tides and stellar mass perturbers. This furnishes a prediction for the relative velocity between the components of a binary and their projected separation. After reviewing recent work evidencing the existence of a critical acceleration scale in Elliptical Galaxies and Globular Clusters, I will show new results showing such a phenomenology in Gaia wide binaries using the latest and most accurate astrometry available. The results are consistent with the Newtonian prediction for projected separations below 7000 AU, but inconsistent with it at larger separations, where accelerations are expected to be lower than the critical a0 value of MONDian gravity. This result challenges Newtonian gravity at low accelerations and shows clearly the appearance of gravitational anomalies of the type usually attributed to dark matter at galactic scales, now at much smaller stellar scales.

Vendredi 19 juillet 2019, 14h00
Salle de l'atelier, Paris
Magnetic fields in Young Protostellar Disks and Jets, and some Astro-chemistry
Chin Fei Lee
ASIAA, Taiwan
résumé :
Magnetic fields in Young Protostellar Disks and Jets, and some Astro-chemistry
Chin Fei Lee, ASIAA Taiwan

I will present our recent ALMA observations towards 3 young protostellar
systems, reporting the possible field morphology in their disks and jets.
In particular, I will report the field morphology implied from the dust
polarizations and SiO line polarizations, and discuss the possible toroidal
and poloidal fields in the disks and jets. I will also discuss the
formation process and the growth of the protostellar disks, and the possible
magnetic braking that reduces the angular momentum in the envelope.
Interestingly, more than 10 organic molecules including prebiotic molecules
are detected in the atmosphere of one of the disks. I will compare their
abundances to those in the hot corinos around low-mass protostars and touch
on the possible formation mechanism of those molecules. Some of the
molecules seem to be formed on CO icy grains and some in the gas phase.
Vendredi 12 juillet 2019, 14h00
Salle de l'atelier, Paris
The chemical structure of planet forming disks
Viviana GUZMÁN
Pontificia Universidad Católica de Chile
résumé :
What sets the composition of nascent planets is a fundamental question in astronomy, and one that is extremely timely considering the large number of exoplanets with very different characteristics that have been discovered in the past years. Whether these planets can host life depends directly on the composition and distribution of the gas where they form, i.e. protoplanetary disks. The recent results of the DSHARP program have shown that sub-structures are an extremely common feature of the dust component disks. Our current efforts are now to characterize the distribution of key organic species in disks. Thanks to the great sensitivity and spatial resolution of current mm-facilities, we can start to understand how and what is the origin of the material that is incorporated into planets.

In this talk I will show ALMA observations of simple species that are very sensitive to photo-chemistry, like carbon chains (C2H and C3H2) and cyanides (CN and HCN, including their isotopologues). HCN is of particular interest as it is a key ingredient in prebiotic chemistry. I will then discuss the current detections of the most complex organics detected in protoplanetary disks so far, like H2CO and CH3OH, key intermediate species in the formation of more complex species in ices, and nitrogen-bearing species, like CH3CN and HC3N. I will finish by commenting on our current efforts to constrain the chemical structure of disks in the planet and comet forming zone.

Vendredi 5 juillet 2019, 14h00
Salle de l'atelier, Paris
Distributions of shock waves: probing extra-galactic turbulence
résumé :
Galactic super-winds driven by stars or supermassive black holes are an important feedback mechanism impacting the formation and evolution of galaxies as well as the enrichment of the intergalactic medium. These multiphase winds are observed at velocities (~1000 km/s) that would completely destroy molecules and ionise atoms if their energy dissipated in simple large scale shocks. An emerging picture instead considers a turbulent cascade mediating the transfer of energy from the large scale to the small, dissipating in myriad lower velocity shocks.

In this context I will present my work on low and intermediate velocity (2-50 km/s) molecular shocks. At low velocities in the dense interstellar medium, the rich complexity of magnetohydrodynamics allows for different kinds of shocks at speeds around the Alfven velocity. Counter intuitively, warm J-type shocks re-emerge at very low velocities which may be important for molecule production in turbulent molecular clouds. At higher velocities, shocks are hot enough to produce significant UV radiation that propagates ahead of the shock to generate a radiative precursor. Such a shock requires a careful treatment of the radiative transfer, and a self-consistent iterative method. I will present my implementation of such methods in the Paris-Durham shock code.
Vendredi 28 juin 2019, 14h00
Salle de l'atelier, Paris
Annulé et reporté
Is accretion-driven turbulence a key process for galaxy growth ?
résumé :
Spitzer and Herschel infrared spectroscopy has revealed a population of nearby galaxies with weak star formation and unusually bright emission lines (e.g. [CII], H2), with very broad linewidths. The line luminosities are greatly in excess of that expected by photoelectric heating of the gas, suggesting that they are powered by the dissipation of turbulent kinetic energy. This discovery of large masses of gas not associated with star formation reveal the potentially important, but largely unexplored, role that turbulence plays in the energetics and formation of multiphase gas on galactic scales. Is this relevant for filamentary gas accretion onto halos of galaxies? I will discuss a toy model in which some of the gravitational potential energy is transferred into gas accretion streams as they penetrate deeper into halos of young galaxies, and part of that energy is dissipated through a turbulent cascade in the warm infalling gas. We have modeled the excitation of the [CII] line as gas is cooling isobarically during its transition from the warm ionized to cold neutral medium. We find that the contribution of [CII] to the total gas cooling rate is increased to 30% and that this [CII] luminosity fraction is largely independent of metallicity. This may explain the recent ALMA detections of [CII] line emission from very high-redshift galaxies, that is not co-spatial with their UV-continuum and have ratios of [CII] to infrared luminosity that are higher than that expected from star formation.
Vendredi 14 juin 2019, 14h00
Salle de l'atelier, Paris
Astronomie au Pôle Sud en 1984 : la mission EMILIE
Jean-Michel LAMARRE
Observatoire de Paris/LERMA
résumé :
En 1984 une équipe du CNRS soutenue par l’INAG (INSU aujourd’hui), en association avec l’Université du Delaware et l’appui du programme antarctique des USA, a installé l’expérience EMILIE pour quelques semaines au Pôle Sud géographique pour y effectuer les premières observations astronomiques en ondes submillimétriques. Les conditions d’altitude et de température font du Pôle Sud un site unique pour ce domaine spectral, mais les conditions d’accès en sont particulièrement difficiles et les questions logistiques y occupent une place vitale, au sens propre.

Le récit de ces premières observations sera replacé dans ce contexte particulier et dans le cadre géographique exceptionnel de ce continent. On s’appuiera pour cela sur les cahiers de manip et sur des photos de la mission, mais on ne se privera pas d’utiliser les images les plus spectaculaires et les données les plus récentes de la recherche polaire, ni d’ailleurs celles de "l’âge héroïque" de l’exploration de l’Antarctique.

L’expérience EMILIE fut pour ses participants une étape importante du long apprentissage technique et scientifique qui a donné naissance au satellite Planck. On doit aussi y voir les prémices de l’observatoire submillimétrique aujourd’hui en activité au Pôle Sud, le « dark sector » de la station Amundsen-Scott.

Vendredi 26 avril 2019, 14h00
Salle de l'atelier, Paris
The Effect of Gravitational Lensing on Cosmological Parameter Estimation
résumé :
A long standing question in cosmology is whether gravitational
lensing by structure biases the apparent distance or the mean flux
density of sources. Interest in this has been rekindled by recent
calculations in 2nd order relativistic perturbation theory which
suggest significant implications for cosmological parameter
estimation from both supernovae and the microwave background. In
this talk I shall first review the somewhat confusing history of
the subject, going back to the early '60s and including both
Weinberg's 1976 argument that there should be no effect on grounds
of flux conservation and the general relativistic "focusing theorem"
of the '80s that seems to contradict this and which foreshadows the
more recent results. I then describe recent work with John Peacock
where we have shown how these conflicts are resolved. Regarding
Weinberg we show that there is a loophole in his argument, and
lensing does in fact bias the apparent distance, but it is a very
small effect. We show that the apparent intrinsic tendency for
structure to cause focusing of light rays that emerges from the
focusing theorem is a statistical rather than a physical effects.
Vendredi 12 avril 2019, 14h00
Salle de l'atelier, Paris
Radiation magnetohydrodynamic models and spectral signatures of plasma flows accreting onto young stellar object
Salvatore COLOMBO
résumé :
According to the largely accepted magnetospheric accretion scenario, classical T Tauri Stars (CTTSs) are young stars that accrete material from their circumstellar disk. The objective of my PhD project is to shed light on the processes governing the physics of the accreting plasma flows, through complete radiation magnetohydrodynamic models. In this talk, I will present the results obtained during my 18 month period in Paris.

First, I will focus on the results obtained from a 3D magnetohydrodynamical (MHD) model of a star-disk system. We simulate the effects of series of flares occurring on the surface of the disk. We observe that each flare produces a hot loops that links the star to the disk; all the loops build up a hot extended corona that irradiates the disk from above. Moreover, the flares trigger overpressure waves that travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity and thus contribute to the mass accretion rate of the star. The accretion columns can be perturbed by the flares. As a result, the streams are highly inhomogeneous, with a complex density structure, and clumped.

Second, I will provide the first assessment of the role of radiation effects on the dynamics and the structure of the impact region of the accreting column onto the stellar surface. In particular, we proved the existence of a radiative precursor in the pre-shock part of the accreting column. To achieve such a result, we have, for the first time, developed a Non Local Thermodynamic Equilibrium (non-LTE) radiation hydrodynamics model, which we implemented in the 3D MHD PLUTO code.”

Vendredi 5 avril 2019, 14h00
Salle de l'atelier, Paris
The magnetized interstellar medium in the Galaxy through Faraday tomography of the radio sky
résumé :
The study of the diffuse Galactic interstellar medium (ISM) is both a
waypoint to investigate the processes that turn gas into stars and to
account for foreground contaminations in modern high-precision
cosmological probes of the Universe.

New structures in the diffuse ionized and magnetized ISM have been
recently observed through Faraday tomography of polarization data at low
radio frequencies. Although the physical origin of these structures
remains uncertain, interesting correlations with tracers of neutral ISM,
such as atomic hydrogen lines and interstellar dust polarization, have
been found. This opens an observational window on the first stages of
phase transition between diffuse/warm and denser/colder gas under the
presence of magnetic fields, allowing us to constrain their role in
structure formation in the ISM.

In my talk I will present an overview of the recent findings in the
diffuse Galactic ISM with the LOFAR radio polarization data. I will
highlight the relevance of a thorough statistical description of these
data both for Galactic studies and for modeling their impact as a
foreground to the detection of the atomic hydrogen 21cm hyperfine
transition from the Epoch of Reionization, a key step with the upcoming
Square Kilometre Array (SKA).
Vendredi 22 mars 2019, 14h00
Salle de l'atelier, Paris
New Planckian quantum phase of the Universe before Inflation: Its present day and Dark Energy implications
Sanchez, Norma
résumé :
The physical history of the Universe is completed by including the quantum planckian and super-planckian phase before Inflation in the Standard Model of the Universe in agreement with observations. In the absence of a complete quantum theory of gravity, we start from quantum physics and its foundational milestone: the universal classical-quantum (or wave-particle) duality, which we extend to gravity and the Planck domain. A new quantum precursor phase of the Universe appears beyond the Planck scale. Relevant cosmological examples as the Cosmic Microwave Background, Inflation and Dark Energy have their precursors in this era. A whole unifying picture for the Universe epochs and their quantum precursors emerges with the cosmological constant as the vacuum energy, entropy and temperature of the Universe, clarifying the so called cosmological constant problem which once more in its rich history needed to be revised. The consequences for the deep universe surveys, and missions like Euclid will be outlined.
Vendredi 15 mars 2019, 14h00
Salle de l'atelier, Paris
A Population of Bona Fide Intermediate-mass Black Holes Identified as Low-luminosity Active Galactic Nuclei
Chilingarian, Igor V.
résumé :
Nearly every massive galaxy harbors a supermassive black hole (SMBH) in its nucleus. SMBH masses are millions to billions of solar mass, and they correlate with properties of spheroids of their host galaxies. While the SMBH growth channels, mergers, and gas accretion are well established, their origin remains uncertain: they could have emerged either from massive “seeds” (10^5–10^6 Msol) formed by direct collapse of gas clouds in the early universe or from smaller (100 Msol) BHs, end products of first stars. The latter channel would leave behind numerous intermediate-mass BHs (IMBHs, 10^2–10^5 Msol). Although many IMBH candidates have been identified, none are accepted as definitive; thus, their very existence is still debated. Using data mining in wide-field sky surveys and applying dedicated analysis to archival and follow-up optical spectra, we identified a sample of 305 IMBH candidates having masses 3× {10}^4 {M} < {M}BH}< 2× {10}^5 {M} , which reside in galaxy centers and are accreting gas that creates characteristic signatures of a type I active galactic nucleus (AGN). We confirmed the AGN nature of 10 sources (including five previously known objects that validate our method) by detecting the X-ray emission from their accretion disks, thus defining the first bona fide sample of IMBHs in galactic nuclei. All IMBH host galaxies possess small bulges and sit on the low-mass extension of the {M}BH}{--}{M}bulge} scaling relation, suggesting that they must have experienced very few if any major mergers over their lifetime. The very existence of nuclear IMBHs supports the stellar-mass seed scenario of the massive BH formation.
Mardi 15 janvier 2019, 11h00
Salle de l'atelier, Paris
ATTENTION jour ET heure inhabituels
Two Thresholds for Globular Cluster Formation and the Common Occurrence of Massive Clusters in the Early Universe
IBM Research Division
résumé :
Young massive clusters (YMCs) are usually accompanied by lower-mass clusters and unbound stars with a total mass equal to several tens times the mass of the YMC. If this was also true when globular clusters (GCs) formed, then their cosmic density implies that most star formation before redshift ~2 made a GC that lasted until today. Star-forming regions had to change after this time for the modern universe to be making very few YMCs. Here we consider the conditions needed for the formation of a ~10^6 Msun cluster. These include a star formation rate inside each independent region that exceeds ~1 Msun/yr to sample the cluster mass function up to such a high mass, and a star formation rate per unit area of Sigma_SFR ~ 1 Msun/kpc^2/yr to get the required high gas surface density from the Kennicutt-Schmidt relation, and therefore the required high pressure from the weight of the gas. High pressures are implied by the virial theorem at cluster densities. The ratio of these two quantities gives the area of a GC-forming region, ~1 kpc^2, and the young stellar mass converted to a cloud mass gives the typical gas surface density of 500-1000 Msun/pc^2. Observations of star-forming clumps in young galaxies are consistent with these numbers, suggesting they formed today's GCs. Observations of the cluster cut-off mass in local galaxies agree with the maximum mass calculated from Sigma_SFR. Metal-poor stellar populations in local dwarf irregular galaxies confirm the dominant role of GC formation in building their young disks.
Vendredi 21 décembre 2018, 14h00
Salle de l'atelier, Paris
Astrochemistry in star forming regions : new modeling approaches
Emeric BRON
résumé :
Star-forming regions present rich infrared and millimeter spectra emitted by the gas exposed to the feedback of young stars. This emission is increasingly used to study the star formation cycle in other galaxies, but results from a complex interplay of physical and chemical processes : chemistry in the gas and on grain surfaces, (de)excitation processes of the atoms and molecules, heating and cooling balance,... Its understanding thus requires detailed astrochemical models that include the couplings between these processes. In this talk, I will present several examples where new modeling approaches of specific processes and their couplings proved crucial to solve persistent observational riddles : from the driving role of UV irradiation in the dynamics of photodissociation regions (PDR) to the efficient reformation of molecular hydrogen in these regions.
Vendredi 7 décembre 2018, 14h00
Salle de l'atelier, Paris
What can Rosetta measurements tell us about the chemical origin of our Solar System ?
Vianney TAQUET
résumé :
The Rosetta spacecraft analysed the Jupiter-family comet 67P/Churyumov-Gerasimenko (67P/C-G) in 2014 and 2015. The ROSINA mass spectrometer on board the Rosetta orbiter carried out a chemical census with unprecedented sensitivity of the coma of 67P/C-G and detected a “zoo” of molecules from simple di-atomic species to complex pre-biotic molecules, such as glycine. The precise abundances together with the signal correlations among species measured by Rosetta/ROSINA now give us invaluable constraints to infer the chemical origin of comet 67P/C-G, and possibly of our Solar System. In this talk, I will take the examples of molecular oxygen and deuterated organic molecules, such as methanol, whose signal correlations and abundance ratios with water ice can be used as powerful chemical tracers. We compared the ROSINA measurements with state-of-the-art astrochemical models applied to dynamical models and considering several scenarios, and with sub-mm interferometric observations of nearby low-mass protostars using the ALMA and NOEMA observatories. The comparison between comet measurements, model predictions and observations of protostars would favour a dark cloud (or “primordial”) grain surface chemistry origin for molecular oxygen and methanol in comets, albeit for slightly warmer and denser dark clouds than those usually considered as solar system progenitors.

Vendredi 30 novembre 2018, 14h00
Salle de l'atelier, Paris
Multiscale star-formation in the Ophiuchus Molecular Cloud: from molecular clouds to brown-dwarfs formation
résumé :
From molecular clouds to stars, every step of the evolution of young stars can be observed in the submillimetric range. The Herschel Space Telescope observed, as part of the Herschel Gould Belt Survey, many molecular clouds. When these molecular clouds are fragmenting, dense prestellar cores accumulating dust and gas are forming and contracting. We performed a census of prestellar dense cores in the Ophiuchus Molecular Cloud, which appear to be coupled with filamentary structures, as part of the paradigm of star-formation inside interstellar filaments. The region was not previously known as filamentary, despite the observation of protostellar alignments. This molecular cloud is under the heavy feedback of active stars nearby seen in the structure of the molecular cloud. Oph B-11, detected with interferometric observations, is a brown dwarf precursor, which final mass will not be important enough for the final star to burn hydrogen. Their formation mechanism is not well constrained, we must find and characterize a first candidate pre-brown dwarf. Oph B-11 was detected along a nearby shock, we characterize chemically. Moreover, higher resolution studies with ALMA show a structured molecular environment and help us constrain the mechanism of formation of this kind of objects. These observations show a series of shocks in different tracers, spatially coincident with the detected position of the pre-brown dwarf, in favour of the gravo- turbulent scenario for the formation of brown dwarfs. I will discuss the legacy of Herschel in the Ophiuchus region in the filament paradigm of star-formation, and the future of these studies with the advent of new instruments, like NIKA2 and its polarimetry facility.
Vendredi 16 novembre 2018, 14h00
Salle de l'atelier, Paris
Quantum walks and astrophysical plasmas
résumé :
Quantum walks (QWs) have been first considered by Feynman in the 1940's and later introduced systematically in the 1990's in the context of quantum information. These discrete automata are a universal quantum computation tool and their first experimental realisation is less than 10 years old (2009).

I will present some of the research conducted at the LERMA since 2012. I will explain that QWs can be viewed as models of Dirac fermions (electrons etc.) interacting with gauge fields like EM fields and gravitation and that QWs can be used to build self-consistent many body theories. These results pave the way towards new numerical simulations and laboratory experiments modelling astrophysical and cosmological relativistic quantum plasmas through QWs.
Vendredi 12 octobre 2018, 14h00
Salle de l'atelier, Paris
Diffuse Molecular Clouds and CO Dark Gas
University of Toledo
résumé :
I provide evidence for describing CO Dark Gas through multi-wavelength analyses of diffuse molecular clouds. A discussion of observations suggesting that diffuse gas represents the envelopes of molecular clouds is presented first. Our understanding of CO in diffuse clouds through a combination of mm-wave, infrared, and ultraviolet measurements is presented, followed by the pictures developed from visible/ultraviolet observations and from longer wavelength data acquired in the Herschel GOT C+ Survey. Our recent synthesis of these results into a unified scheme is described, where self-consistent physical conditions (temperature and density) for diffuse molecular (CO Dark Gas) are summarized.
Vendredi 5 octobre 2018, 14h00
Salle de l'atelier, Paris
A Survey of Atomic Carbon [CI] in High-redshift Main Sequence Galaxies
Cosmic Dawn Center, Copenhagen
résumé :
I will present the first results of an ALMA survey of the lower fine structure line of atomic carbon [C I](^3 P1 ^-3 P0 ) in far infrared-selected galaxies on the main sequence at z ~ 1.2 in the COSMOS field. This sample is further detected in several far-IR and sub-mm bands and one or multiple CO transitions, allowing for a comprehensive comparison with starbursting systems and quasars reported in the literature so far. We show that the [CI] luminosity correlates on global scales with the infrared luminosity LIR similarly to low-J CO transitions. We report a systematic variation of L’[CI] /LIR as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and sub-millimetre galaxies at fixed LIR. The ratios of [CI], low-J CO transitions, and dust continuum luminosities are similar for main-sequence galaxies and for local and high-redshift starbursts, suggesting that [CI] is a good tracer of molecular gas mass as CO and dust in all these systems. Moreover, we estimate the neutral atomic carbon abundance, the fundamental ingredient to calibrate [CI] as a gas tracer. We find lower [C I] abundances in main- sequence galaxies than in starbursting systems and sub-millimetre galaxies, as a consequence of the canonical alpha-CO and gas-to-dust conversion factors. This argues against the application to different galaxy populations of a universal standard [CI] abundance derived from highly biased samples.

Vendredi 28 septembre 2018, 14h00
Salle de l'atelier, Paris
The [CII] emission line as a molecular gas mass tracer in galaxies at low and high redshift
résumé :
So far the gas conditions in main-sequence galaxies at the peak of the cosmic star formation history have been mainly investigated through the CO emission lines. However, observing the CO transitions at higher redshift becomes challenging, since the lines luminosity weakens as metallicity decreases. A powerful alternative could be the [CII] emission at 158um instead: it is one of the brightest lines in the far IR regime observed in star-forming galaxies and it is the main coolant of the interstellar medium. Local studies show that the [CII] luminosity correlates with the galaxy star formation rate (SFR), although main-sequence sources and starbursts seem to have different behaviours. At higher redshift the picture is even less clear and only samples of starbursts have been analyzed so far. To remedy this situation we have observed with ALMA a sample of 10 main-sequence sources at z ~ 2 and we complemented our sample with literature data at lower and higher redshift. We found that the [CII] luminosity correlates with galaxies' molecular gas mass, independently of their depletion time, metallicity, and redshift. This lays foundations for future explorations of the interstellar medium of starbursts and galaxies at much higher redshift (z > 4).

Vendredi 21 septembre 2018, 14h00
Salle de l'atelier, Paris
Understanding the structure of molecular clouds: Multi-line wide-field imaging of Orion B
résumé :
The new generation of wide-bandwidth high-resolution receivers turns
almost any radio observation into a spectral survey. In the case of
wide-field imaging of the interstellar medium, such a wealth of data
provides new diagnostic tools, but also poses new challenges in terms of
data processing and analysis.

The ORION-B project aims at observing 5 square degrees of the Orion B
molecular cloud, or about half of the cloud's surface, over the entire
3mm band. The emission of tens of molecular tracers have been mapped,
including CO isotopologues, HCO+, CN, HNC, N2H+, methanol, SO, CN...
Machine learning techniques have been applied to these maps, in order to
segment the molecular cloud into typical regions based on their
molecular emission, and to idenfify the most meaningful correlations of
different molecular tracers with each other and with physical quantities
such as density or dust temperature.

The spatial coverage, together with the spatial and spectral resolution,
also allow to characterize statistically the kinematics and dynamics of
the gas. The amount of momentum in the compressive and solenoidal
(rotational) modes of turbulence are retrieved, showing that the cloud
is dominated by solenoidal motions, with the compressive modes being
concentrated in two star-forming regions - which is in line with the
overall very low star formation efficiency of the cloud, and highlights
the role of compressive forcing in the star formation process. The
filamentary network of the molecular cloud also proves to have
particluarly low densities, and is very stable against gravitational
collapse and fragmentation, which also points at a young evolutionary
stage of the filaments.
Vendredi 6 juillet 2018, 14h00
Salle de l'atelier, Paris
Accretion - ejection : from young stars up to AGN
Observatoire de Paris/LUTH
résumé :
Accretion and ejection of matter is a process that plays a central role in the physics of Young Stellar Objects as well as of super-massive Black Holes in Active Galactic Nuclei. For both cases semi-analytical solutions can be obtained via a non-linear separation of the variables, which are the spherical distance and the magnetic flux function, in the relevant physical quantities. Using a self-similar solution as initial conditions, we performed 2.5D magneto-hydrodynamical simulations of the accretion-ejection system for Classical T Tauri Stars, YSO of about one solar mass. The collimated stellar jet and the accreting magnetosphere attain a quasi steady state in only few stellar rotations, confirming the robustness and stability of the self-similar solutions. Depending upon the accretion velocity, the density in the accretion column and the static magnetosphere, coronal episodic mass ejection can result from the interaction between the jet and the accretion column.

Spine jet modelling for relativistic outflows aroud Kerr Black Holes has been performed by applying meridional self-similarity technics to GRMHD equations for a perfect conducting ideal gas. We are able to describe the outflow near the super-massive central BH, and in particular to study the effects of the BH rotation. The model, characterized by 8 parameters, is based on a first order expansion of the governing general relativistic equations in the magnetic flux function around the symmetry axis of the system. I will present four enthalpy driven solutions with different field geometries and Lorentz factors, wherein the contribution of the Poynting flux is rather small. The jet power of the ultra-relativistic outflow solutions are of the same order as the one determined from numerical simulations conducted by several groups. Furthermore, our model is able to describe an incoming flow entering the BH horizon since at the stagnation radius, pairs are created from neutrinos or highly energetic photons coming from the disk. Coupling inflow and outflow models allows us to describe the MHD flow from the horizon of the BH up to infinity. We can estimate the different contributions of each of those processes: the energetic component at the BH horizon coming from the Blandford-Znajek effect or the generalized Penrose mechanism , and the energetic input due to the creation of pairs.

Vendredi 29 juin 2018, 14h00
Salle de l'atelier, Paris
Réseaux de neurones profonds et données astronomiques, un premier retour d’expérience
INRIA Nançy Grand Est
résumé :
Nous présenterons deux approches s’appuyant sur les réseaux de neurones profonds. La première approche utilise des méthodes de détection d’objets afin d'identifier de nouvelles nébuleuses planétaires à partir du catalogue IPHAS (The INT Photometric H_alpha Survey of the Northern Galactic Plane). La deuxième approche s’appuie quand à elle sur les travaux récents autour des auto-encodeurs variationnels qui utilisent un réseau de neurones pour l’apprentissage non supervisé de caractéristiques discriminantes. Cette approche est utilisée dans le cadre de l’estimation des formes 3D d’astéroïdes à partir des images radars en provenance des observatoires de Goldstone et Arecibo.

Vendredi 8 juin 2018, 14h00
Salle de l'atelier, Paris
Planet-Hunting with ALMA
University of Michgan
résumé :
The Atacama Large Millimeter Array is revolutionizing our understanding of planet formation. Today we now have detected numerous disks that exhibit symmetric gaps in the emission distribution from sub-mm/mm sized dust particles present in the disk midplane. These gaps have traditionally between posited as being carved by the presence of hidden planets. I will review the techniques and pitfalls that are used to infer the presence of planets within these systems which rely on difficult to constrain assumptions regarding scaling factors such as the dust to gas ratio or local molecular abundance. More directly, I will present a new technique that relies on measuring velocity residuals in the emission of CO isotopologues that deviate from Keplerian rotation with 2 m/s accuracy. In the HD 163296 disk, these residuals directly trace gradients in the gas pressure across the gaps, without need for a priori knowledge of absolute scaling factors. 2D and 3D hydrodyamical models with planets embedded deep within the gaps beautifully replicate the detected structure in the velocity residuals. This provides the strongest evidence to date that unseen Jupiter mass planets are present in these Myr-old systems. I will discuss the bright future of this technique and outline methods that might be used to further confirm the presence of young planets. One possibility that I will explore is the use of disk chemistry. Here, I will present a 3D physical/chemical model that includes two point sources: star and accreting protoplanet. With generic assumptions based on planet formation theory and observations, we find that the localized heating of an accreting protoplanet can alter the chemistry in its near vicinity by, for example, releasing volatiles that otherwise would be frozen on grain surfaces. Thus, if planets are accreting gas these effects will be present and are predicted to be detectable. In all, we are on the cusp of new era of bringing submm-wave astronomy into the realm of planet detection.

Lundi 4 juin 2018, 14h00
Salle de l'atelier, Paris
Jour de la semaine exceptionnel
Inefficient jet-induced star formation in Centaurus A: High resolution ALMA observations of the northern filaments
Quentin SALOME
UNAM, Mexico
résumé :
Star formation is one of the key mechanisms driving the evolution of galaxies across cosmic times. The physical properties and the multi-scale dynamics of the molecular gas influence the star formation efficiency. The environment certainly also plays a role in star formation. In particular, recent studies suggest that AGN can regulate the gas accretion and thus slow down star formation. However, evidence of AGN positive feedback is also invoked in a few radio galaxies.

I will present different studies of the northern filaments of Centaurus A at different resolutions. These filaments extend on scales up to 15 kpc, aligned with the radio-jet, and show evidence of recent star formation (Rejkuba et al. 2001). They are the perfect testbed region for positive feedback, here through jet-induced star formation.

At the intersection of the radio jet and one of the HI shells that surround the galaxy (Schiminovich et al. 1994), CO emission in the shell has been detected with SEST (Charmandaris et al. 2000). With APEX, we mapped the CO emission along the FUV filaments that lie at the jet-HI interaction. In particular, we discovered a large amount of molecular gas outside the HI gas (Salomé et al. 2016). However, while the molecular gas reservoir is important, it is very inefficient to form stars compared to star-forming disc galaxies. To understand why star formation is inefficient, we obtained ALMA observations to map the CO emission along the filaments, at a resolution of ~20 pc (Salomé et al. 2017). Such resolution enabled us to separate giant molecular clouds and study their physical properties (mass, size, velocity dispersion).
Vendredi 1 juin 2018, 14h00
Salle de l'atelier, Paris
Protoplanetary disks at high angular resolution
résumé :
With ALMA and high-contrast optical/IR imaging, protoplanetary disks are revealed to be structured objects. They display rings, spirals, vortices and warps. These structures appear to be extremely well-defined and often have high contrast. This poses the question: what processes cause these conspicuous structures? Are these signs of planet formation? Or do they betray the existence of just-born planets in these disks? In this talk I will discuss these observations and some theoretical models that attempt to explain them. I will show that these structures indicate that dust “pebbles” are being moved around and are trapped in so-called “pressure traps”. I will show that planetary/substellar companions perturb the disk, but that also disk-internal processes can explain some of the ringlike dust traps. I will discuss some ideas to explain the strong warps seen in some of these protoplanetary disks. Finally I will give a preview of the results of an ALMA Large Programme on 20 resolved protoplanetary disks.
Vendredi 25 mai 2018, 14h00
Bâtiment B (Salle RdC proche cantine), Paris
Closing the gap between simulations and observations
résumé :
In this talk I will give an overview over my latest results to compare
current observations with latest 3D radiation MHD simulations of turbulent
protoplanetary disks. I will focus on the dust thermal emission in
protoplanetary disks, covering the outer regions which emit in the sub/mm.

In the talk I will present comparisons between the expected dust scale
height from simulations and observations and how we could use this results to learn about the gas disk dynamics.
I will also show the possibility to explain mm polarization by dust grains
which are aligned by the magnetic field.

Finally I will draw conclusion on the results and discuss about the
possibility to observe MRI activity with current telescope facilities.
Mercredi 16 mai 2018, 11h00
Salle de l'atelier, Paris
Jour exceptionnel et heure exceptionnelle
Unveiling the Cosmic Dawn with the 21 cm signal
résumé :
" During the first billion years of the universe, the intergalactic
medium undergoes the process of reionization, triggered by the formation
of the first stars and galaxies. During this extended process, the
remaining neutral patches of hydrogen emit in the 21-cm line. The
resulting signal should allow us to build a continuous 3-dimensionnal
map of the universe that encode a wealth of information about
reionization (topology, nature of the sources, population, etc...).
Observing the 21-cm signal is the goal of a number of existing
radio-telescopes and one the the key science goals of the upcoming SKA.
Then, an intensive modelling effort is required to decode the
information and put constraints on astrophysical processes.

After giving the general context, I will briefly present and comment on
the recent possible first detection of the signal with the EDGES
instruments (Bowman et al., 2018). Then I will described our recent work
on modelling the signal using detailed numerical simulations, and our
work on using machine learning techniques to derive constraints on
astrophysical parameters from the observed signal."
Vendredi 11 mai 2018, 14h00
Salle de l'atelier, Paris
Bright Compact Bulges (BCBs) at intermediate redshifts
IUCAA, Pune, India
résumé :
Studying bright, intermediate-redshift, disc dominated galaxies in Chandra Deep Field South, in optical and infrared rest-frame wavelengths, we found a new class of bulges which is brighter and more compact than ellipticals. We refer to them as “Bright, Compact Bulges” (BCBs) - they resemble neither classical nor pseudo-bulges and constitute ~ 12% of the total bulge population at these redshifts. Interestingly, we find that only ~ 0.2% of the bulges can be classified as BCBs in the local Universe. Although disc galaxies with BCBs host the most massive and dominant bulge type, their specific star formation rate is 1.5-2 times higher than other disc galaxies. This is contrary to the expectations that a massive compact bulge would lead to lower star formation rates. We speculate that our BCB host disc galaxies are descendant of massive, compact and passive elliptical galaxies observed at higher redshifts. Those high redshift ellipticals lack local counterparts and possibly evolved by acquiring a compact disc around them. The overall properties of BCBs supports a picture of galaxy assembly in which younger discs are being accreted around massive pre-existing spheroids.
Vendredi 6 avril 2018, 14h00
Salle de l'atelier, Paris
Realistic dust properties for dust growth in dense cores
Charlène Lefèvre
IRAM, Grenoble
résumé :
Grain growth is expected from far infrared observations but the emission process implies a degeneracy between dust properties and temperatures. On the contrary, the scattering phenomenon does not depend on temperature but relies on dust geometry (size and shape). Nowadays, no dust grain model is able to reproduce consistently the multi-wavelength observations of dense cores including both scattering and emission. Recent laboratory works show the impact of dust composition on emissivities. Dust grain models from the literature suffer from a lack of flexibility: either the dust composition is fixed and too constraining for dense cores (with a magnesium over iron ratio too low) or the dust size distribution is set for given density, porosity, and coagulation time, that likely differ from one core to the other.

Based on methods used to derive aggregate properties for protoplanetary disks, I calculate dust properties for dense cores that take into account dust composition, size distribution and shape. I will present a fast method used to derive realistic dust properties for dense cores in a short computation time. In particular, I will illustrate the impact of porosity on scattering efficiency and dust emissivity. I will also review the consequences on dust amount, and show that grain growth requests cold dust in starless cores. Finally, I will discuss the coagulation time needed to build large dust aggregates.
Vendredi 23 mars 2018, 14h00
Salle de l'atelier, Paris
New Statistical Evidence for AGN-Feedback Giving Rise to ‘Mass-Quenching’ in Central Galaxies
ETH, Zurich
résumé :
One of the most striking features of the population of local galaxies is that the distributions of several key galaxy properties are highly bimodal (e.g. color and star formation rate). In general, high mass galaxies in dense environments, with bulge-dominated morphologies and pressure supported kinematics are more frequently passive (non-star forming) than lower mass galaxies in low density environments, with disc-dominated morphologies and rotationally supported kinematics. Understanding which, if any, of these correlations is causally related to the ‘quenching’ of star formation in galaxies remains an active and hotly debated area of investigation in modern astrophysics. Theoretically, a wealth of physical processes have been evoked to account for central galaxy quenching, including halo mass quenching from virial shocks, feedback from active galactic nuclei (AGN; in either the quasar or radio mode), stabilizing torques from central mass concentrations, or even magnetic fields interacting with the hot gas halo. I will present strong new statistical evidence which suggests that the quenched fraction of local central galaxies is primarily related to their central kinematics (Bluck et al. 2016; 2018 in prep.). I will show that this is broadly consistent with quenching from AGN feedback, through a detailed comparison with a semi-analytic model and a cosmological hydrodynamical simulation.

Vendredi 2 mars 2018, 14h00
Salle de l'atelier, Paris
Machine Learning in Astronomy
University College London
résumé :
Astrophysics, High Energy Physics and other Physical sciences are undergoing an ‘industrial revolution’ with huge data sets and complex models to describe them. For example, galaxy surveys (e.g. DES, DESI, LSST, Euclid, SKA) will catalog billions of galaxies over the coming decade. Similarly, Particle Physics experiments, e.g. at CERN, generate gigantic data sets. Such data sets call for entirely new approaches for data analysis. Fortunately, powerful ‘Machine Learning’ methods have been invented by statisticians and computer scientists and they are widely applied across science, technology and medicine.
The talk will illustrate examples such as classification of galaxies and supernovae, estimating galaxy redshifts and other properties from their colours, and deriving the mass of the Local Group.
We shall also discuss programmes for training of the next generation of data scientists in connection with industry partners, e.g. UCL’s Centre for Doctoral Training In Data Intensive Science http://www.hep.ucl.ac.uk/cdt-dis/
Vendredi 16 février 2018, 14h00
Salle de l'atelier, Paris
Refining our Understanding of the Merger History of Galaxies
University of Nottingham
résumé :
Baryonic assembly of galaxies is one of the largest questions in extragalactic studies, which relates to many other issues, including environment, feedback, star formation, gas accretion and merging. In fact, all of these processes are related and must be accounted for and understood to paint a full picture of galaxy assembly. Perhaps the most straightforward of these processes is measuring the merging and star formation histories. I will present results from the REFINE (Redshift Evolution and Formation in Extragalactic Systems) project, combining in a new reanalysis of the three deepest and large NIR surveys take to date: UDS, Ultra-VISTA and VIDEO. Using consistently measured stellar masses and photometric redshifts for galaxies in these fields up to z =3, I will show how the major and minor merger rate can be consistently measured across these fields. Our new method involves a full use of the PDF for photo-zs and stellar masses. We show how the merger fraction and rate are lower than previous results and the implications for this for other methods of galaxy assembly and feedback mechanisms.

Vendredi 2 février 2018, 14h00
Salle de l'atelier, Paris
La matière organique des comètes est-elle celle des Diffuse Interstellar Bands ?
LATMOS, Université de Versailles Saint-Quentin
résumé :
L’analyse des poussières du noyau de la comète 67/P montre qu’elles
sont constituées presque pour moitié de grosses molécules organiques
(en masse, 45% organique, 55% minéral, Bardyn et al. 2017).
Nous suggérons que ce sont les mêmes molécules qui produisent,
dans le milieu interstellaire (ISM), les absorptions diffuses
dans les spectres stellaires (DIBs, Diffuse Interstellar Bands).
Ces grosses molécules qui provoquent les DIBs étaient certainement
présentes dans la parcelle d’ISM qui s’est condensée pour former
le proto-système solaire. Nous suggérons qu’elles se sont
conservées dans le processus de formation du noyau cométaire.
Selon le scénario établi par Davidsson et al. (2016) et fondé
sur de nombreuses preuves recueillies au cours de la mission
Rosetta il s’agit d’une accrétion hiérarchique douce, partant
des grains interstellaires en plus gros grains, jusqu’à la
taille actuelle du noyau. A l’inverse, la grande variété d’une
comète à l’autre du rapport D/H dans la glace d’eau montre que
celle-ci a dû se sublimer au moment de la formation du système
solaire, et donc également le manteau glacé des grains.

Nous appuyons notre suggestion sur des arguments quantitatifs et
qualitatifs. D’une part une estimation du rapport matière
organique/minéral (dérivé de l’analyse des DIBs) d’au moins
RISM=0.32, à comparer à environ 0.8 pour la comète (hors glace).
D’autre part, le sondage des nuages interstellaires montre que
quand la ligne de visée s’approche du centre, les absorptions par
les DIBs plafonnent, alors que le gradient de la loi de rougissement
dans l’UV augmente, signe de présence des très petits grains.
Les molécules organiques pourraient s’agglomérer pour participer
à la formation des grains qui se retrouveront dans le nuage
primordial puis la comète. Cette conclusion implique qu’une mission
de retour d’échantillon cométaire aurait de l’intérêt non seulement
pour les comètes, mais aussi pour le Milieu Interstellaire.

Cet exposé reprend dans ses grandes lignes l’article Bertaux
et Lallement (MNRAS, 469, S646–S660 (2017).
Vendredi 26 janvier 2018, 14h00
Salle de l'atelier, Paris
A prescription to regulate star formation by turbulence and impact on the galaxy stellar mass assembly at high redshift.
Morgane, COUSIN
LAM, Marseille
résumé :
The star formation mechanism is one of the most complex processes in
galaxy evolution. It resides in non-linear density, velocity and
magnetic field regions. Nowadays, the regulation of the star formation
is always one of the key points of galaxy formation models. In high
resolution hydrodynamics simulations or in semi-analytical models a
large set of prescriptions have been invoked to constrain star
formation: supernovae energy injection, activity of a super-massive
black-hole. Despite all these regulation processes, galaxy formation and
evolution models fail to reproduce the stellar formation history. In
this talk, I will present the new semi-analytic model G.A.S.. In this
new model, in addition to standard feedback recipes, two new physical
prescriptions are introduced: the inertial turbulent cascade, and the
growth of thermal instabilities in the hot halo phase. I will present
the stellar mass function predicted by this new model from z = 6 to z =
1.0. In the second part of the talk, I will focus on a specific
application of this new model: Predictions of the [CII] 158 microns line
emission in high-redshift galaxies.
Vendredi 19 janvier 2018, 14h00
Bâtiment B (Salle RdC proche cantine), Paris
lieu exceptionnel
Molecular gas reservoirs during the winding-down of star formation
Université Hébraïque de Jerusalem
résumé :
Star formation in the Universe decreased by an order of magnitude in the last ten billion years. Probing the molecular gas reservoirs from which stars are formed is crucial to understand this winding-down of star formation. The PHIBSS2 legacy program at the IRAM NOEMA interferometer surveys the molecular gas properties of galaxies at different redshifts on and around the main sequence, where most of star formation occurs. It notably contributes to show that the cosmic evolution of the star formation rate is mainly driven by that of the molecular gas fraction, albeit with a small inflexion of the star formation efficiency. At z=0.5-0.8, we observe the CO(2-1) line for 61 targets, from which we determine molecular gas masses, gas fractions and depletion times. We separately obtain disk sizes and bulge-to-total luminosity ratios, which enable us to characterize morphological changes. The Kennicutt-Schmidt relation between molecular gas and star formation rate surface densities is strikingly linear, which argues in favor of uniform star formation processes across cosmic time. Our results also suggest an ongoing supply of molecular gas to compensate for star formation and a contribution of disk stars to the growing bulge.
Vendredi 12 janvier 2018, 14h00
Salle de l'atelier, Paris
Just how abundant are fullerenes in space?
IAC, Canaries
résumé :
Theoretical work, astronomical observations and laboratory measurements
of fullerenes and their hydrogenated forms (fulleranes) provide evidence
that these molecules could be responsible for diffuse interstellar
bands, the 217.5 nm UV bump (a prominent feature in the extinction
curves observed in many lines of sight of our Galaxy and other
galaxies), and even on the intriguing anomalous microwave emission
discovered in several regions of star formation, in molecular clouds and
HII regions. The detection of C60 and C70 fullerenes in planetary
nebulae in our Galaxy and in the Magellanic Clouds and detection of the
C60+ cation in diffuse interstellar clouds reinforce the hypothesis that
these molecules are common in the interstellar medium and could indeed
be significant contributors to at least some of these processes. I will
also report on new detections of fullerenes in star-forming regions.
These molecules should be considered in reaction networks in
protoplanetary disc chemistry as they may have a role on the generation
of prebiotic molecules.
Vendredi 1 décembre 2017, 14h00
Salle de l'atelier, Paris
Theoretical modeling of dust evolution in galaxies
ASIAA, Taiwan
résumé :
The evolution of dust in the interstellar medium is one of the
most important aspects in galaxy evolution. For the evolution of dust,
it is important to clarify the dust amount and the dust properties
(especially the grain size distribution). We modeled the evolution of
these two aspects in a consistent framework and proposed the following
dust evolution scenario: Dust formation in stellar ejecta is important
for the dust enrichment in the early stage of galaxy evolution or in the
metal-poor phase. The subsequent evolutions of dust amount and grain
size distribution are governed by interstellar processing of dust.
Especially, grain disruption by shattering and dust growth by accretion
enhance the abundance of small grains while coagulation pushes those
created small grains to large sizes. We also show that the above
processes governing dust evolution have a variety of impacts on some
observable quantities such as extinction curves. I also talk about our
recent efforts of implementing the above dust evolution processes into
hydrodynamical galaxy evolution simulations. Through a simulation of a
single galaxy, we clarified the evolution of spatial dust distribution
and spatial variation of extinction curves. We have also implemented
dust evolution into a cosmological simulation, based on which we are
able to predict the dust evolution over a cosmological time and volume.
Vendredi 24 novembre 2017, 14h00
Salle de l'atelier, Paris
The SPHINX simulations : the first billion years and cosmic reionisation
résumé :
The epoch of reionisation marks a major shift from a cold neutral Universe to a warm ionised one, a transition which was thought to be powered by UV radiation emitted from young massive stars in the first galaxies.

Our understanding of this epoch is still very limited: observationally we glimpse a handful of the most luminous galaxies existing at the end of the epoch, but with the advent of the James Webb Telescope and other upcoming instruments we will soon start getting statistical information about the sources powering reionisation. Theoretically, the best way to gain an understanding is with cosmological simulations. However, those are very expensive, so either people simulate large patches of the reionising Universe without resolving the galactic sources, or go for tiny patches where the stellar sources inside one or a few galaxies are resolved, but the large-scale process is lost. I will present a new suite of simulations, called the Sphinx project, where we have developed new methods that allow us to perform radiation-hydrodynamical simulations of cosmic reionisation, resolving the emission and escape of radiation in the inter-stellar medium of hundreds of galaxies all evolving together in the same patch of the Universe, hence capturing the interplay of small- and large-scale processes.

I will describe the simulations and the key developments that make them possible, and show results from a pilot Sphinx study looking into the surprising impact on reionisation of spectral energy distribution (SED) models containing binary populations, compared to more classical SEDs.
Mardi 3 octobre 2017, 14h00
Salle des séminaire de l'IAP, Paris
Exceptionnellement, seminaire LERMA-IAP, Mardi à l'IAP
The MUSE Hubble Deep Field Survey
Roland BACON
CRAL, Lyon
résumé :
The MUSE Hubble Deep Field Survey is deepest spectroscopic
survey ever performed over the entire Ultra Deep Field (UDF) area. It
provides ~1700 spectroscopic redshifts, an order of magnitude more
compared to the data that has been accumulated on the UDF over the
past decade, up to AB 30 in magnitude and 6.7 in redshift. The depth
and high quality of the data enables new and detailed studies of the
physical properties of the galaxy population and their environments
over a large redshift range. In this talk I will show in advance of
publication a number of important results achieved by the survey on a
diversity of topics: investigation of bias in photometric redshift,
spatially resolved stellar kinematics at z~1, properties of CIII
emitters, FeII emission in stellar forming galaxies, Lya luminosity
function and its impact for reiniozation, Lya extreme EW objects
undetected in UDF, Lya extended halos, evolution of the galaxy major
merger rate with redshift, etc.
Vendredi 22 septembre 2017, 14h00
Salle de l'atelier, Paris
Cosmology with galaxy clusters: The Sunyaev-Zel’dovich effects to probe the formation of distant clusters
résumé :
Clusters of galaxies represent the last step of the formation of large scale structures in the Universe. They are both fundamental for cosmology, and unique astrophysical laboratories. The clusters grow from the accretion of surrounding structures and from the mergers of subclusters in the most energetic events since the Big Bang. One way to study these objects is to measure their imprint in the Cosmic Microwave Background from the Sunyaev-Zel'dovich (SZ) effect.

After introducing the key role of clusters in our understanding of the co-evolution of the dark matter and the baryons, I will discuss how the SZ effect can be used to probe the gas pressure and velocity to understand how they form. I will then present state-of-the art SZ observations, which allow in particular to obtain the first image of the moving gas in a galaxy cluster.
Vendredi 30 juin 2017, 14h00
Salle de l'atelier, Paris
White Dwarfs, Gaia, and the Age of the Galaxy
Ted von HIPPEL
résumé :
What is the star-formation history of the Milky Way? How old are Galactic halo
and thick disk stars? Traditional age-dating of stars relies on clusters, which
only offer a limited view of these stellar populations. I will show that white
dwarf stars offer a way forward. Specifically, I will show how optical and
near-IR photometry, Gaia astrometry, and Bayesian modeling allow us to determine
high precision ages for individual white dwarfs and derive population ages.
Lundi 19 juin 2017, 14h00
Salle de l'atelier, Paris
A Wide-Area Submillimeter Imaging and Spectroscopy Survey with the Chajnantor Sub/millimeter Survey Telescope
résumé :
We are developing a project to build a 30-m telescope operating at wavelengths as short as 850 µm with 1 degree field of view for imaging and multi-object spectroscopic surveys. With this project, we aim to: trace the evolution of dusty, star-forming galaxies from high redshift to the z ~ 1-3 epoch when they dominate the cosmic star formation rate; connect this population to the high-redshift rest-frame UV/optical galaxy population; use these dusty galaxies, the most biased overdensities, to guide ultra-deep followup at z > 3.5 and possibly z > 7; map the fragmentation structure of molecular clouds, its connection to the stellar initial mass function, and the episodic accretion onto protostellar cores; determine how the rate and efficiency of star formation in the Milky Way and nearby galaxies depends on stellar mass, local environment, and overall galaxy morphology; characterize the evolution of galaxy clusters via mergers, accretion, and energy injection by correlating gas pressure, internal motions, and temperature with other probes; and use galaxy clusters to measure the universe’s peculiar velocity field. We will describe the expected surveys addressing these science goals, the novel telescope design, and the planned survey instrumentation.
Vendredi 10 mars 2017, 14h00
Salle de l'atelier, Paris
Interstellar magnetic fields: radio is the future
IRAP, Toulouse
résumé :
Magnetism is a fundamental force of nature. Since interstellar magnetic fields were discovered in our Galaxy in 1949, it has been established that magnetism is pervasive in the Universe. Throughout the years we have inferred the properties of galactic magnetic fields from a variety of observational methods. Not only from observations but also from a theoretical point of view, it became clear that magnetic fields impact the spatial distribution, dynamics and energetics of the interstellar medium.
In this talk I will start by reviewing the classical observational methods of interstellar magnetic fields and what they have taught us about our Galaxy. Then I will present a new method, known as Faraday tomography. This powerful technique, now being applied to radio data from e.g. LOFAR, is revealing a bewildering variety of structures in the Galactic diffuse interstellar medium. While exploiting these new radio data - a true gold mine - we are already looking ahead to SKA and to what this cutting-edge radio telescope will disclose on our Galaxy.
Mardi 21 février 2017, 14h00
Salle RdC Batiment B (à côté cafeteria), Paris
The Las Campanas Stellar Library: an essential tool to interpret NIR spectra of galaxies
résumé :
We present the most comprehensive up-to-date intermediate resolution (R=6500) stellar library, which covers the entire near-infrared wavelength range (0.83 to 2.5um). It contains spectra of 1000+ stars across the HR diagram collected with the FIRE spectrograph at the 6.5m Magellan Baade telescope processed with a dedicated bright source data reduction pipeline. Among others, our library includes about 250 AGB stars, 50 LMC/SMC stars, and also a sample of chemically peculiar stars. We have about 150 stars in common with the X-Shooter Spectral Library project which will allow us to perform cross-checks, assess the data quality and pin down potential systematic problems. By the start of the JWST mission we plan to produce a new generation of stellar population models which will enable the extragalactic community to interpret nearby galaxy spectra in the NIR and analyze unresolved stellar populations in a manner similar to the one used in the optical domain. The spectral resolution will be sufficient to study internal kinematics and stellar content of dwarf galaxies and nuclear star clusters in nearby galaxies. For about 650 stars, we built complete optical-to-NIR spectra at R=6500 using re-calibrated optical spectra from INDO-US and UVES-POP stellar libraries.

If I have time, apart from the stellar library, I will also talk about some aspects of the ground-based NIR spectroscopy in general, which are very important for the data analysis and interpretation but are usually known only to people working very closely to instruments/telescope operations.
Vendredi 3 février 2017, 14h00
Salle de l'atelier, Paris
High-mass star and cluster formation in the Milky Way
Frédérique MOTTE
résumé :
Herschel revealed high-density cloud filaments, which are forming clusters of OB-type stars. Given their high star formation activity, these so-called mini-starburst cloud ridges could be seen as “miniature and instant models” of starburst galaxies. The characteristics of mini-starburst ridges challenge star formation models and could soon shed light on the origin of massive clusters.

In one of these ridges, the star formation efficiency increases with volume gas density and contradicts statistical models of star formation rates (SFR). Moreover, its measured core mass distribution suggests that the initial mass function (IMF) may not be determined, in these extreme environments, at the prestellar stage.
Vendredi 27 janvier 2017, 14h00
Salle de l'atelier, Paris
Warm and hot gas around low-mass protostars:disentangling its origin, and connecting it to warm and hot gas in distant galaxies
Institut Niels Bohr, Copenhagen
résumé :
Low-mass protostars are energetic little beasts. As they accrete matter from the surrounding gas cloud, they launch jets and bipolar outflows that drive shocks into the natal environment, both on small (100 AU) and large (> 10^4 AU) scales. These feedback processes naturally heat the gas, making it directly observable at far-infrared wavelengths. Thanks to the incredibly successful Herchel mission, we now have a detailed picture of where this warm and hot gas is, and what the underlying processes are that govern the heating and cooling. I will discuss these, before touching on how we can use observations of nearby low-mass protostars to quantify heating and cooling processes in distant galaxies, as well as outlining how we can trace the low-mass population in these galaxies.
Vendredi 25 novembre 2016, 14h00
Salle de l'atelier, Paris
Cluster science in the era of Euclid, LSST, J-PAS and SKA
Begoña ASCAS0
résumé :
Galaxy clusters, as the largest structures of the universe, are both
spectacular laboratories for galaxy studies and powerful cosmological
probes. The advent of next-generation surveys will provide hundreds of
thousands of cluster and group detections that will serve the basis for a
wide range of analysis.

This talk will describe some recent work performed to obtain accurate
cluster selection functions for three optical/IR next-generation surveys:
Euclid, LSST and J-PAS, based on cosmological simulations specially
designed to be realistic and provide consistent comparisons. These results
will be complemented with preliminary results obtained for the future radio
experiment SKA and compared with similar results coming from future X-ray
and SZ experiments.
Vendredi 21 octobre 2016, 14h00
Salle de l'atelier, Paris
Dust evolution from diffuse to dense ISM
Nathalie YSARD
résumé :
What are the properties of dust in the ISM and how do they change depending on local density? Since dust properties influence, for example, the formation and temperature of the major molecules in dense clouds and the grain dynamical behaviour when forming stars and protoplanetary disks, it is important to characterise the grain size, structure, shape, and material composition in all phases of the ISM.

SEDs of dense ISM clouds show a decrease in colour temperature, and an increase in spectral index and opacity in the far-IR and sub-mm. These variations cannot be explained with environmental differences alone, but are assumed to occur due to changes in the dust properties. Recently, Planck-HFI data revealed that dust properties do not change only in the transition from diffuse to dense ISM but also inside the diffuse ISM itself, which was considered rather homogeneous until then. This finding has a great impact on our understanding of the diffuse ISM since it implies that the dust properties vary.

In the context of THEMIS (The Heterogeneous Evolution dust Model of Interstellar Solids), a recent core-mantle dust model, we show that all observations of the dense ISM can be rather well explained by accretion and coagulation processes, whereas variations in grain mantle thickness and size distribution can account for the variations observed in the diffuse ISM.
Vendredi 14 octobre 2016, 14h00
Salle de l'atelier, Paris
Probing the Beginnings of Planetary Birth in the Age of ALMA: Snowlines and Hydrocarbon Rings
University of Michigan
résumé :
The Atacama Large Millimeter Array (ALMA) has begun operations in the high desert of northern Chile offering unprecedented spatial resolution and sensitivity within the millimeter and sub-millimeter atmospheric windows. The study of planet birth is one of the key science areas enabled by ALMA due to the ability to resolve both gas and dust emission within the planet formation zones of young gas-rich circumstellar disks. This is highlighted by the fantastic high resolution images of HL Tau and TW Hya showing significant structure in the emission from pebbles within young and disks. In this talk I will explore the related physics and chemistry of gas-rich disks and emphasize new breakthroughs in our understanding brought about by ALMA in concert with data from the Herschel Space Observatory. In particular I will report on the physical/chemical links in terms of snow-lines and the likely
formation of pebbles and possibly planetesimals. Snow-lines represent chemical transitions (ice to vapor) in the disk and have long been posited as favorable sites for planet formation. With ALMA we have now directly and indirectly resolved the carbon monoxide snow-line on the disk surface in several disk systems. Here I will present the first detection of an optically thin isotopologue of carbon monoxide that is directly probing the gas surface density profile and chemistry of the disk midplane to radii that lie within the natal region of giant planet birth. Beyond the CO snowline we also detect the presence of striking rings in the emission of hydrocarbons, which we
posit is linked to overall carbon and oxygen chemistry throughout the disk. We discuss an emerging consensus where we are likely observing the implantation of key C, H, O, and N volatiles into pre-planetary materials which produces an
apparent radial gradient in the carbon to oxygen content in the gas and solids. This elemental abundance gradient will likely be imprinted within the atmospheres of forming gas giants and sets constraints on the location of the volatile reservoir needed to form habitable terrestrial worlds.
Vendredi 7 octobre 2016, 14h00
Salle de l'atelier, Paris
HI-to-H2 transitions in turbulent ISM
Shmuel BIALY
Tel Aviv University
résumé :
Conversion of hydrogen gas from atomic (HI) to molecular (H2) form is of critical importance for the evolution of the interstellar medium (ISM) and for star-formation in galaxies. H2 is also a must ingredient for the formation of other molecules such as CO, OH and H2O, that serve as coolants and observational tracers.
I will discuss an analytic theory of the HI-to-H2 transition in photon-dominated regions, and present analytic HI/H2 density profiles.I will discuss the effects of turbulence on the HI-to-H2 transition and the HI column density probability distribution function (PDF). I will compare theory with 21 cm observations of the Perseus molecular cloud and of W43 - resembling quiet VS active star-forming regions.
Vendredi 30 septembre 2016, 14h00
Salle de l'atelier, Paris
Galaxy evolution from the gas perspective
Tobias BROWN
Swinburne University
résumé :
The importance of cold gas in the picture of galaxy evolution is well known, as is its role as a probe of recent environmental effects on galaxies. However, sensitivity limitations mean the extent to which environment impacts the gas-star formation cycle of galaxies remains unclear. With this talk I will show how we take full advantage of the powerful HI spectral stacking technique to overcome this obstacle and quantify the gas content for the entire gas-poor to -rich regime. This was accomplished using an the largest sample of HI and multi-wavelength information available (28,000 galaxies), selected according to stellar mass (M*>10^9 Msol) and redshift (0.02 <= z <= 0.05). I will present HI scaling relations with key structural, star formation and environmental metrics, using stacking to provide strong observational evidence of significant and systematic environment driven gas suppression across the group regime, well before galaxies enter the cluster. Furthermore, I will show that gas depletion is more closely associated to halo mass than local density and cannot be reproduced by starvation of the gas supply alone, invoking systematic ram-pressure stripping of the cold gas to explain this. Finally, I will show preliminary results highlighting the role of HI in regulating the correlation between stellar mass, star formation and gas-phase metal abundance known as the “fundamental" mass-metallicity relation.
Vendredi 23 septembre 2016, 14h00
Salle de l'atelier, Paris
The satellite population of the Milky Way
Institute of Astronomy, Cambridge
résumé :
The diffuse stellar halo of the Milky Way only contributes a small fraction of
the total light of the Galaxy but, comprising the oldest and most metal poor
stellar population, it offers a glimpse into the early stages of galaxy
formation. Model predictions regarding the predicted amount of substructure
that should be present in Galaxy-sized halos are currently very uncertain,
therefore, careful observational constraints on the characteristics of our own
halo can be very valuable to constrain cosmological models. In this talk, I
will present our updated version of an algorithm used to systematically search
for Milky Way satellites using photometric surveys. This will allow us to
quantify the luminosity function of the Milky Way satellites, very important
for Milky Way modelling, and has lead to the discovery of two new interesting
satellites: Crater 2, a diffuse giant galaxy, the fourth larger satellite of
the Milky Way, and Aquarius 2, which is only discovered by pushing the
detection limits of current surveys.

Vendredi 9 septembre 2016, 14h00
Salle de l'atelier, Paris
Airborne Astronomy with SOFIA: Motivation and Science Highlights
résumé :
SOFIA, short for Stratospheric Observatory for Infrared Astronomy,
is a 2.7m telescope flying on a Boeing 747SP at altitudes of 12-14km,
to detect and study mid- and far-infrared radiation that is blocked
by water vapor in the earth's atmosphere and cannot reach the ground.
It is the successor to the Kuiper Airborne Observatory and currently
the only access to and platform for astronomical observations in the
far-infrared (30-300 microns), except for balloon-borne telescopes.

Although a bilateral project (80:20)
between USA (NASA/USRA) and Germany (DLR/DSI), it is open for proposals
from the world-wide astronomical community at large.
It addresses many science questions that Herschel has left unanswered
and offers observational opportunities similar to and beyond Herschel.

SOFIA has reached full operational capability in 2014 and has just
completed its Cycle 5 call for proposals,
offering a suite of 5 mid- and far-infrared imagers and spectrometers.
Recent instruments to be commissioned (2nd generation) and offered in
shared risk (in Cycle 4 and 5) are upGREAT (a 14 pixel THz array)
and HAWC+ (a 4x32x40 FIR imager with a polarimetric observing mode).

SOFIA is new window to the local universe to study fundamental processes
in astrophysics and astrochemistry, such as ISM heating and cooling, star
formation, and feedback processes, as well as age-dating star forming
molecular cores using new chemical molecular clocks.
Science questions include molecular cloud formation and destruction,
the dynamics of cloud collapse and circumstellar disks,
protostellar outflows, turbulence, shocks and magnetic fields.
Emission and absorption of fine structure lines ([CII] and [OI]),
highly excitated CO rotational lines, the use of HD (as a proxy for H2),
and other molecular tracers (e.g. OH, OD, or H2D+) will enlighten us
about complex but key physical interactions in interstellar space.

In my presentation, I will describe a glimpse of SOFIA science
highlights and discoveries in its first few years of operation.
I also plan to discuss SOFIA's unique potential as the only successor
to the Herschel far-infrared satellite (2009-2013).

SOFIA normally flies out of California, but once a year also deploys
to the Southern Hemisphere (currently to New Zealand).

Jeudi 8 septembre 2016, 11h00
Salle de l'atelier, Paris
Central 300 pc of the Galactic center probed by the infrared spectrum of H3+
Takeshi OKA
Enrico Fermi Institute, Chicago
résumé :
The Galactic center’s Central Molecular Zone (CMZ), a region of ~300 pc centered on the super-massive black hole Sgr A*, is the center of astrophysical activities rich in extraordinary phenomena and has long been a focus of studies from the radio and IR to X-rays and Gamma-rays. This region which is less than 10^-5 of the Galaxy in volume is estimated to contain 10 % of molecules in the Galaxy. The richness of molecules has made radio emission of CO, CS, NH3, HCN etc. and radio absorption of OH and H2CO powerful probes to study the CMZ.

It was found in 1999 using the United Kingdom Infrared Telescope that this region contains enormous amount of H3+ [1]. Subsequent observations have revealed 100 % surface filling factor of H3+ and rich velocity profiles with high dispersion. The abundance and ubiquity of H3+ together with the presence of many bright infrared stars deeply embedded in the CMZ have made H3+ a powerful general probe to study the environment of the CMZ.

The discovery by the Subaru Telescope of H3+ in the (J,K) = (3,3) metastable level 361 K above the ground (1,1) level demonstrated high temperature of the environment [2]. An analysis of the H3+ spectrum observed in the Gemini South Observatory toward the star GCS 3-2 of the Quintuplet Cluster, the brightest infrared star in the CMZ, based on a model calculation of thermalization [3], has revealed the presence of a large column length of warm (T ~ 250 K) and diffuse (n < 100 cm^-3) gas and high H2 cosmic ray ionization rate (zeta ~ 5 × 10^-15 s^-1) [4]. The same gas has been detected toward 8 stars in the central 30 pc of the CMZ [5]. These observations used stars which were already studied by IR star specialists.

In 2008 we launched a program to find stars that are useful for the H3+ spectroscopy to extend our observations from the central 30 pc to 300 pc. Starting from a few million stars listed in the Spitzer Space Telescope GLIMPSE Catalog, we chose a few thousand bright stars and, after dropping obvious red giants, we examined ~ 500 stars by low resolution IR spectrum. This immediately bore fruits for 2 stars [6] but most observations toward ~ 30 stars are being written up now [7]. In the meantime we were extremely happy to have analysis of our column densities from other group [8]. The most recent results and a comparison of the analyses will be discussed.

[1] Geballe, T.R., McCall, B.J., Hinkle, K.H., Oka, T. 1999, ApJ, 510, 251
[2] Goto, M., McCall, B.J., Geballe, T.R.,…., Oka, T. 2002, PASJ, 54, 951
[3] Oka, T., Epp. E. 2004, ApJ, 613, 349
[4] Oka, T., Geballe, T.R., Goto, M., Usuda, T., McCall, B.J. 2005, ApJ, 632, 882
[5] Goto, M., Usuda, T., Nagata, T., Geballe, T.R., McCall, B.J.,….Oka, T. 2008, ApJ, 688, 306
[6] Geballe, T.R., Oka, T. 2010, ApJ, 709, L70
[7] Oka, T., Geballe, T.R., Goto, M., Usuda, T., Indriolo, N. two papers in preparation.
[8] Le Petit, F., Ruaud, M., Bron, E., Godard, B., Roueff, E., Languignon, D., Le Bourlot, J. 2016, A&A, 585, A103
7 août 2016, 11h00
Salle de l'atelier, Paris
jour et heure inhabituels
Enrico Fermi Institute, Chicago
Vendredi 22 juillet 2016, 10h30
Salle DANJON, Paris
Terahertz Heterodyne Arrays for Astronomy
résumé :
The lifecycle of clouds of gas and dust in the Interstellar Medium (ISM) can be studied through THz spectral lines.
Using the high spectral resolution afforded by heterodyne instruments, astronomers can disentangle the filamentary structure and kinematics within these clouds creating a need to develop large format THz heterodyne arrays.
Building on the heterodyne array, SuperCam, a 64-pixel receiver at 345 GHz for CO, new technologies to produce an integrated 4x1-pixel heterodyne array receiver at 1.9 THz for the astrophysically important [CII] line are under development and testing at JPL.
A compact coherent 4x1-pixel multiplier chain for use as a local oscillator (LO) is quasi-optically coupled to a 4x1-pixel mixer block with superconducting hot electron bolometer (HEB) mixers.
This prototype can be used to build larger focal plane arrays and can also be scaled in size to create arrays at other THz frequencies.
Vendredi 24 juin 2016, 14h00
Salle de l'atelier, Paris
Quantitative Polarimetry: from Star Formation to Cosmological Studies
CITA, University of Toronto
résumé :
We are entering a golden age of multiwavelength polarimetry with numerous CMB experiments (e.g., SPIDER, BICEP/Keck, LiteBIRD) hunting for primordial gravitational waves through B-mode polarization, and a dozen of big instruments designed to elucidate the roles of magnetic fields in star formation (e.g., SOFIA, SMA, ALMA, EVLA). The correct determination of B-mode signal, as well as solid understanding of magnetic fields in star formation, are only achieved when we have a quantitative treatment of dust polarization. In this talk, I will present first my results on quantifying the polarization spectrum of a newly discovered emission component, namely anomalous microwave emission. Second, I will review the quantitative, predictive theory of grain alignment based on radiative torques, and discuss ab-initio modeling of dust polarization that incorporates well-tested physics and comparisons with observational data. Finally, I will discuss the unique potential of quantitative polarimetry in the next decade to elucidate the roles of magnetic fields in star and planet formation, and to reveal fundamental properties of magnetic turbulence and cosmic dust, through bridging solid theory, numerical simulations with observations.
Vendredi 20 mai 2016, 14h00
Salle de l'atelier, Paris
UV photoprocessing in astrochemistry
Leiden University
résumé :
In many astrophysical environments chemical bond-breaking is driven by ultraviolet radiation. Understanding and quantifying UV photodissociation, ionisation, desorption, and photofragment branching ratios for the many atoms and molecules present in space is the subject of fundamental studies in laboratory and theoretical chemical physics. I will discuss these processes and their application to the molecules observed in interstellar clouds, protoplanetary disks, and planetary atmospheres; and some detailed laboratory studies of N2 and CO photodissociation.
Vendredi 13 mai 2016, 14h00
Salle de l'atelier, Paris
The regulation of star formation on galactic scales
CEA, Saclay
résumé :
Galaxies form the bulk of their stellar mass on very long timescales, except during rare « starburst » phases. The slowness and inefficiency of the gas-to-star conversion process is a major puzzle for galaxy formation models, which systematically predict star formation histories more rapid than observed, resulting in unrealistic stellar masses and gas depletion. Feedback mechanisms, such as supernovae explosions or active nuclei, are often invoked as key processes to regulate star formation. Modern hydrodynamic simulations reach a sufficient resolution and accuracy to model to cold, dense and turbulent phases of the multi-phase ISM, in which stars form and feedback mechanisms propagate their effect. They demonstrate that feedback is responsible for the regulation of the total masses of galaxies, through the expulsion of huge amounts of baryons, but do not strongly affect the efficiency of star formation. Instead, hydrodynamic and gravitational instabilities play a direct role in regulating star formation: they drive the collapse of a small fraction of interstellar reservoirs into star-forming cores, but at the same time they generate high turbulent pressures that largely regulate the collapse of gas into stars. The self-regulated turbulence cascade naturally accounts for the typical inefficiency of star formation, and the transition to efficient starburst phases during galaxy collisions. While this physics remains poorly described in cosmological simulations, it tends to reconcile the standard cosmological model with observations of galaxy formation.

Vendredi 29 avril 2016, 14h00
Salle de l'atelier, Paris
Mapping the temperature and density toward Maffei 2 using formaldehyde emission
Monica Rodriguez
Observatoire de Paris/LERMA
résumé :
I will present a study aiming to determine the physical characteristics of the molecular gas traced with the formaldehyde molecule (H2CO) in the starburst galaxy Maffei 2. For this, we used both IRAM facilities; the 30m single dish and the Plateau de Bure interferometer to map three H2CO lines at 218.22 GHz, 218.76 GHz and at 145.60 GHz toward Maffei 2. The line ratios between these lines are proved to be a good tracer of the temperature and density in Galactic molecular clouds and in external galaxies. I will show the results and give a preliminary
analysis of these H2CO observations.
Vendredi 22 avril 2016, 14h00
Salle de l'atelier, Paris
Intensity mapping of the atomic Carbon [CII] line: a new observational promise of the post-reionization and reionization epoch
Guilaine LAGACHE
LAM, Marseille
résumé :
The fine structure line [CII] at 158 microns is one of the brightest
emission lines in the spectra of galaxies. It is considered to be the
dominant coolant for neutral atomic gas in the interstellar medium.
Conveniently, [CII] is redshifted into the relatively transparent
sub-millimeter and millimeter atmospheric windows for 4.5<z<9. At high
redshift, [CII] is thus a unique tracer of galaxy formation and an
excellent probe of the epoch of reionization (z>6). However, so far,
[CII] studies of very distant galaxies have been limited, with detection
of only a handful of galaxies. The full power of ALMA will revolutionize
the field, detecting [CII] in individual galaxies in the heart of the
reionization era. However, while ALMA will probably follow-up hundreds
of high-redshift galaxies, its small field of view and narrow bandwidth
will strongly limit its sensitivity/efficiency to conduct large unbiased
spectral line surveys. Rather than detecting [CII] in selected galaxies,
[CII] intensity mapping, by measuring fluctuations on large regions of
the sky in a wide frequency band, is a new observational promise.

In this talk, I will discuss how [CII] intensity mapping measurements
can offer a straightforward alternative for tracing the large structure
of galaxies at z>4.5. By measuring [CII] fluctuations, I will show how
we can obtain unprecedented constraints on star formation and dust
build-up at a key epoch in the Universe. I will also discuss how
cross-correlation of the signals (e.g. [CII] and galaxy surveys or HI),
as well as the lower redshift (z<2.5) CO-line fluctuations, could be
used to further understand the physics of galaxy formation and
reionization. Finally I will present the CONCERTO experiment, a [CII]
spectrometer capable of covering tens of square degrees with a high
sensivitity and proposed as a new intensity mapping experiment.
Vendredi 18 mars 2016, 14h00
Salle de l'atelier, Paris
Magneto is not a hero: The Galactic magnetic field and the formation of molecular clouds.
Juan Diego SOLER
CEA, irfu
résumé :
ESA's Planck satellite has produced the first all-sky map of the polarized emission from dust at submillimetre wavelengths. Compared with earlier ground-based and balloon-borne observations this survey is an immense step forward in sensitivity, coverage, and statistics. It provides new insight into the structure of the Galactic magnetic field, as well as the first statistical characterization of one of the main foregrounds to CMB polarization.

I will summarize the main results of the Galactic magnetic field studies by the Planck Consortium, particularly focusing on the correlation between the magnetic field and the column density structures, its relation to previous studies based on the observation of starlight polarization and the Zeeman effect, and its implications for our understanding of the formation of density structures in the interstellar medium.
Vendredi 26 février 2016, 14h00
Salle de l'atelier, Paris
La mission Rosetta en orbite autour de la comète 67P/Churyumov-Gerasimenko
Nicolas BIVER
Observatoire de Paris, LESIA
résumé :
Partie le 2 mars 2004, la mission Rosetta accompagne la comète 67P/Churyumov-Gerasimenko dans son périple autour du Soleil depuis Aout 2014. Elle a déposé le module Philae à sa surface le 12 novembre 2014 qui, après quelques péripéties, nous a donné les premières informations depuis la surface d'une comète. Mais la mission Rosetta ne se terminera qu'en septembre 2016 et elle continue à suivre l'évolution de la comète sur plus de la moitié de son orbite. La comète 67P est passée au périhélie le 13 aout 2015 à 1.24 UA du Soleil et nous avons pu suivre la montée puis la baisse de son activité. Je passerai en revue les premiers résultats obtenus par la mission ainsi que les différentes phases de cette mission qui n'a aucun égal pour ce qui est de sa complexité avec les autres missions d'exploration planétaire.
Vendredi 12 février 2016, 14h00
Salle de l'atelier, Paris
Radio Loud AGNs and the distant Universe. From Super Massive Black holes to high-redshift galaxy clusters
Observatoire de Nice
résumé :

In my talk I will present my research work to exploit the Radio Loud Active Galactic Nucleus (RLAGN) subclasses of Low Luminosity Radio Galaxies (LLRGs) and blazars to study the properties of RLAGNs from sub-parsec to Mega-parsec scales.
I will first focus on my recently developed Poisson Probability Method (PPM) to search for high-redshift (z ? 1?2) galaxy clusters around LLRGs. I use a sample of 32 LLRGs within the COSMOS field and demonstrate that ~70% of them live in rich groups and clusters. This rate is in agreement with the fraction found for low redshift LLRGs and it is significantly higher than that for high power radio galaxies. Cluster candidates found with our method are excellent targets for present observatories such as VLT and ALMA as well as next generation space telescopes such as James Webb Space Telescope (JWST). The method reveals itself as a powerful tool to search for distant galaxy clusters in ongoing and forthcoming photometric surveys such as DES, LSST, and Euclid. I will then describe the complete sample of blazars that I have built. They belong to the flat-spectrum radio quasar (FSRQ) subclass from the WMAP 7yr point source catalog and within the SDSS area. The sample comprises 80 objects flux limited at 1 Jy at 23 GHz, 69% of which have a clearly detected ‘blue bump’ in the rest-frame optical-UV Spectral Energy Distribution (SED). I exploit the bump to perform super massive black hole (SMBH) mass estimates of quality competitive with the best alternative methods. I also find evidence of a dusty circum-nuclear torus in the FSRQs of the sample by means of multiwavelength radio-to-UV SED modeling. The detection of infrared torus emission is possible under special conditions (e.g. low-frequency synchrotron peak, high torus-to-disc luminosity ratio).

Mercredi 13 janvier 2016, 11h00
Salle de l'atelier, Paris
Jour et heure inaccoutumés
Cosmic dust and laboratory astrophysics: the Nanocosmos project
Christine JOBLIN
IRAP, Univ. Toulouse III/ CNRS
résumé :
Dust is a key component in the evolution of astronomical objects, from regions of star and planet formation to galaxies. Models of these regions require a proper description of the microphysical processes involving dust, photons, gas and their interactions. Studying these processes has however to face our poor knowledge of the detailed composition and structure of these grains. An additional difficulty is to study these processes under the physical conditions found in space. Solving these difficulties is at the heart of laboratory astrophysics.

In this seminar, I will describe our methodology in studying very small carbon-rich particles, from model particles to more realistic ones. As an example, we have shown that polycyclic aromatic hydrocarbons (PAHs), the smallest in size dust particles, are produced in photodissociation regions by photoevaporation of very small grains, which could be PAH clusters. However, observations suggest more complex grains of mixed aromatic/aliphatic composition. More insights into the formation and evolution of these grains have therefore to be searched for.

The new instruments of astronomy such as the interferometer ALMA open the possibility to investigate the physical and chemical conditions that lead to dust formation in evolved stars. In the laboratory, one can then build reactors in which these conditions are approached and realistic dust analogues can be made. The particles can then be injected into chambers where they can be studied under interstellar conditions. Such methodology is now at the heart of the Nanocosmos project in which astrophysicists, physicists and chemists have joined their efforts to investigate the formation and evolution of cosmic dust.

The Nanocosmos ERC Synergy project (2014-2020) is led by the three PIs: J. Cernicharo and J. A. Martín-Gago (ICMM, CSIC, Madrid) and C. Joblin (IRAP, CNRS, Toulouse). Several research teams and institutes are involved on both sides. In Toulouse, IRAP, LCAR, LCPQ and LAPLACE laboratories carry the activity.
Vendredi 11 décembre 2015, 14h00
Salle de l'atelier, Paris
Cometary Observations : from Halley to Tchury
LERMA, Paris
résumé :
Mankind has always been fascinated by comets. A brief review of early comet observations will be presented. The highlights of the fanatastic results obtained by Rosetta and Philae half a billion Kilometers away from the Earth show that the European Space Agency and its partners have made an unprecedented technological advance. The most recent observations will be reported.
Vendredi 4 décembre 2015, 14h00
Salle de l'atelier, Paris
Modelling the Milky Way in the Gaia era
Observatoire de Paris, GEPI
résumé :
In this talk, I will review some of the most recent discoveries about our Galaxy and discuss why they challenge our view of the galactic stellar populations, their mutual links and their evolution.
I will also discuss how the exceptional amount of data that Gaia, and related spectroscopic surveys, will soon deliver, will help in understanding the way our Galaxy formed and evolved.
Vendredi 20 novembre 2015, 14h00
Salle de l'atelier, Paris
Interstellar Isotope Ratios Reveal Details of Chemical Evolution in the Present Epoch
Univ. of Toledo, Ohio
résumé :
Present day nucleosynthesis enriches the interstellar medium with newly synthesized products. I will present recent results on the lithium and nitrogen isotope ratios from observations of diffuse molecular clouds. Two interstellar environments, the star-forming region IC 348 and the supernova remnant IC 443, show evidence for relatively recent production of Li-6. For the other sight lines in the study, the typical Li-7/Li-6 ratio is about 8 and is apparently lower than the meteoritic value of 12. Thus, the mix of sources seems to have changed since the Solar System formed. In a similar vein, we derive a local N-14/N-15 ratio of 274(18), consistent with other determinations of local gas, but much lower than the results from Genesis of 441(6). These nitrogen results bear on models of chemical fractionation in molecular clouds.
Vendredi 23 octobre 2015, 14h00
Salle de l'atelier, Paris
Are Supermassive Black Holes of stellar black hole origin ?
Institut d’Astrophysique de Paris,
résumé :
One of the most puzzling question in Galaxy Evolution and Cosmology is the origin of Super Massive Black Holes (SMBH), reaching up to ~10**9 Msol at cosmological distances. Stellar masses of 10**12 Msol and metal abundances Z=~0.01 are measured for z=4 template radio galaxies by best-fitting their continuous (HST, Spitzer, ISO, Herschel and submm) Spectral Energy Distributions (SEDs) with Pégase.3 star formation laws.

As a direct consequence, the corresponding mass of stellar black holes is measured from the cumulated supernovae already exploded at the galaxy age t, by subtraction of ejecta to the initial supernova mass. The result is ~10**9 Msol of stellar black holes and neutron stars, surprisingly comparable to the SMBH masses measured from the SDSS catalogs. Of similar dense degenerated matter as SMBH, respecting the metal and SEDs observations, stellar black holes would migrate towards the galaxy core by dynamical friction, merely explaining the birth and growth of SMBH (Rocca-Volmerange et al, 2013, 2015). Consequences on cosmology, galaxy evolution and the link of semi-analytic and numerical models of galaxy formation are discussed.
Mercredi 30 septembre 2015, 09h00
Salle de l'atelier, Paris
Journée scientifique du LERMA “Hautes énergies” (VHE) et “Milieu Interstellaire” (MIS)
organisée par Antoine GUSDORF
résumé :
Les synergies envisageables entre communautés scientifiques “Hautes énergies” (VHE) et “Milieu Interstellaire” (MIS), sur la base de l’utilisation conjointe de CTA et ALMA-NOEMA, tant du point de vue scientifique que de celui de la gestion du flux de données.

Programme preliminaire:

Multi-wavelengths studies:

- GRBs and their high energy emission with CTA

S. Vergani (GEPI, OP)

- Tracking the fuel for star formation in GRB host galaxies

E. Le Floc’h (CEA)

- Radio-jets studies, AGN multi-wavelength studies

O. Hervet (LUTH, OP)

CRs acceleration:

- The radio to high energy link in PWNe and SNRs

F. Acero (LUPM/CEA)

- Assessing the interstellar content in old SNRs

A. Gusdorf, (LERMA, OP)

- Energetic particle acceleration in proto-stars

A. Marcowith (LUPM, Montpellier)

CRs influence on the ISM:

- CRs diffusion towards molecular clouds

S. Gabici (APC, associated to OP)

- CR induced ionisation of molecular clouds close to supernova remnants
P. Hily-Blant (IPAG)

- CRs in lensed galaxies: C+/C/CO, H2O diagnoses

P. Guillard (IAP)

- The treatment of CRs in the PDR Meudon code

B. Godard (LERMA, OP)

Technical aspects:

- Handling ALMA data

P. Salomé (LERMA, OP) or R. Moreno (LESIA, OP)

- Handling CTA data

M. Servillat or C. Boisson (LUTH, OP)

- The NOEMA project

F. Gueth (IPAG)
Vendredi 10 juillet 2015, 14h00
Salle de l'atelier, Paris
Galaxy Formation in Warm Dark Matter Cosmology
Nicola MENCI
INAF, Osservatorio di Roma
résumé :
I will present the effects of assuming a Warm Dark Matter
(WDM) power spectrum in cosmological models of galaxy formation, and
the impact of such an assumption on the statistical properties of
galaxies, including their star formation histories, their color
properties, the abundance of substructures and the evolution of their
luminosity functions. By comparing the results with observations, I will
show how galaxy formation models can provide robust constraints on the mass
of the assumed WDM candidate.
Mardi 7 juillet 2015, 14h00
IAP, salle des séminaires (entresol), Paris
jour inaccoutumé : mardi & salle inaccoutumée : IAP
Research at the ASIAA and the Greenland Telescope Project.
Paul Ho
Academia Sinica Institute of Astronomy and Astrophysics
résumé :
Research at the ASIAA are focused on the radio and OIR efforts. In the optical path, we are collaborating on building advanced instruments for CFHT, Subaru, and TMT. We are also developing the TAOS-2 project at San Pedro de Martir in Mexico. In the radio path, we have been working on AMiBA, SMA, and ALMA. The latest effort is in deploying the Greenland Telescope to the Summit Station in Greenland in order to achieve the highest angular resolution in astronomy. In this talk, I will emphasize more on the submm wavelength part of our developments.
Vendredi 3 juillet 2015, 14h00
Salle de l'atelier, Paris
Tracing the evolution of galaxies and structure in the universe with GISMO
Johannes G. STAGUHN
NASA, Goddard
résumé :
The GISMO 2 mm bolometer camera at the IRAM 30m telescope has been
available to the astronomical community for many years. For studies of the local universe,
GISMO provides observational capabilities across a wide range of astronomical sources,
including observations of galactic dust, free-free emission associated with molecular clouds near HII
regions, and synchrotron radiation from supernova remnants as well as magnetic structures in the Galactic center region. However, the observational capabilities of GISMO are optimized for studies of the
evolution of galaxies and structure in the universe. At very high
redshifts these goals can be achieved through deep survey observations,
while at medium redshifts spatially resolved observations of the SZ
decrement in galaxy clusters provide important clues on the formation of
structure throughout cosmic times.

Following a brief overview over some science highlights from nearby
sources, I will present the latest results from our deep surveys in
GOODS-N, COSMOS, followed by a summary of our results from SZ
observations of the Planck cluster PLCK G147.3-16.6.
Jeudi 25 juin 2015, 14h00
Salle de l'atelier, Paris
jour inaccoutumé
S7: Probing the Physics of Active Nuclei
Michael DOPITA
Mt Stromlo Observatory
résumé :
The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is providing
integral field spectroscopy on ~140 nearby active galaxies in the wavelength region
3300-7000Å at resolutions as high as 7000. In this talk, I present the first results
of this survey. I show how, by combining data analysis with detailed photoionisation
and shock modelling, we are gaining a new understanding of the physics of both Seyfert
and LINER galaxies.
Vendredi 12 juin 2015, 14h00
Salle de l'atelier, Paris
Galactic Distribution of [C II] and its Relationship to Star Formation
William D. Langer
résumé :
The fine structure line of ionized carbon, [C II], at 1.9 THz, is a tracer of many phases of the interstellar medium and has been proposed as a probe of the star formation rate in galaxies across cosmic time. The HIFI instrument on Herschel has made possible for the first time a study of the distribution of this gas probe in the Milky Way. I will review the findings of the Herschel Open Time Key Programme, Galactic Observations of Terahertz C+, a [C II] survey of the properties and structure of the Milky Way, as well as the utility of [C II] as a probe of star formation in the Galaxy and external galaxies, including AGNs.
Mercredi 27 mai 2015, 14h00
Salle de l'atelier, Paris
Tracing the evolution of galaxies and structure in the universe with GISMO
>Johannes G. STAGUHN
résumé :
The GISMO 2 mm bolometer camera at the IRAM 30m telescope has been
available to the astronomical community for many years. For studies of the local universe, GISMO provides observational capabilities across a wide range of astronomical sources, including observations of galactic dust, free-free emission associated with molecular clouds near HII
regions, and synchrotron radiation from supernova remnants as well as magnetic structures in the Galactic center region. However, the
observational capabilities of GISMO are optimized for studies of the
evolution of galaxies and structure in the universe. At very high
redshifts these goals can be achieved through deep survey observations,
while at medium redshifts spatially resolved observations of the SZ
decrement in galaxy clusters provide important clues on the formation of
structure throughout cosmic times.

Following a brief overview over some science highlights from nearby
sources, I will present the latest results from our deep surveys in
GOODS-N, COSMOS, followed by a summary of our results from SZ
observations of the Planck cluster PLCK G147.3-16.6.
Vendredi 22 mai 2015, 14h00
Salle de l'atelier, Paris
Mass loss and time-dependent chemistry in the AGB star IRC+10216
résumé :
IRC+10216/CW Leo is the closest high mass loss AGB star (D ? 130 pc). It
is surrounded by a spherical envelope with an exceptionally rich
molecular content. Over 80 molecular species have been detected in this
envelope, which have largely contributed to our understanding of
circumstellar (CS) and interstellar (IS) chemistry. IRC+10216’s outer
envelope expands radially at a nearly uniform velocity (14. 5 km/s, or
1’’ per 50 years). These remarkable properties and the envelope large
angular size make it a unique probe of time-dependent chemistry over
timescales of thousands of years.

I will present high spatial resolution PdBI, SMA and ALMA observations
that shed light on the mass loss process, which has built up the thick
CS envelope, and that call into question the current chemical models.
Mardi 5 mai 2015, 14h00
Salle de l'atelier, Paris
jour de la semaine inaccoutumé
Volatiles in Protoplanetary Disks and the C/N Budgets of Terrestrial Worlds
résumé :
Compared to the Sun, the C/Si and N/Si ratios of the bulk silicate Earth are depleted by four to five orders of magnitude. What sets this depletion, and how might it vary in exoplanetary systems? In the core accretion model of terrestrial planet formation, the fate of volatile species depends on both their initial speciation in the protoplanetary disk and chemical/physical processing in planetesimals and planetary-scale bodies. This talk will examine the distribution of C-, N- and O-containing volatiles in the disks around young stars, using a combination of infrared, far-infrared and mm-wave observations, and discuss the delivery to and fate of such volatiles on nascent planetary surfaces.

Vendredi 10 avril 2015, 14h00
Salle de l'atelier, Paris
Radial migration in galactic disks and its impact on the chemical evolution of the Milky Way
résumé :
I will present an overview of the potential impact of radial migration
on the chemical evolution of the Milky Way disk, based on the results
of recent studies with N-body and semi-analytical models. I will
discuss, in particular, the impact on the age-metallicity relation and
its dispersion and the chemical properties of the thin and thick disk.
Vendredi 27 mars 2015, 14h00
Salle de l'atelier, Paris
Complex molecules towards star-forming regions
Cécile FAVRE
University of Michigan
résumé :
One of the most important questions in the astrophysics of the interstellar medium (ISM) is whether, how, when, and where complex organic molecules (i.e. molecules that contain 6 or more atoms), including prebiotic species, are formed. And in the Interstellar-Earth connection context, could this have a bearing on the origin of life on Earth? Formation mechanisms of complex organic species, which include potentially prebiotic molecules, are still much debated and may include grain-mantle chemistry and/or gas-phase chemistry. The analysis of the emission of the interstellar molecules gives strong insight into the understanding of the physical conditions (temperature, density) and chemistry that are occurring in star forming regions.

I will present some notable results on the detection of complex molecules, such as methyl formate, ethyl cyanide and glycolaldehyde (pre-sugar), towards low- and high-mass star forming regions. They are based on (sub)millimeter single-dish (APEX, IRAM-30m, Herschel) and/or interferometric (PdBI, SMA, ALMA) observations of ISM sources.
Vendredi 20 mars 2015, 14h00
Salle de l'atelier, Paris
Radiative Feedback in the Interstellar Medium
Samuel GEEN
CEA, Saclay
résumé :
A great deal of debate has already occurred in the literature concerning
how efficient supernova feedback is at shaping the ISM, but relatively
little work has been carried out on the influence of feedback processes
that occur before a massive star reaches the end of its life. I will
discuss two sets of simulations aimed at quantifying our understanding
of how these processes, chiefly UV photoionisation, can shape the
environment around massive stars and drive the subsequent evolution of
the ISM.
Vendredi 20 février 2015, 14h00
Salle de l'atelier, Paris
High angular resolution Sunyaev-Zel'dovich observations with the NIKA camera
Barbara COMIS
LPSC, IN2P3, Grenoble
résumé :
As the last step of the hierarchical structure formation process, clusters of galaxies represent the
largest gravitationally bound objects that we can observe in our Universe. Since they formed all
along the cosmic history, they contain plenty of information about the evolution of the Universe,
and can then provide a strong tool for cosmological investigation, complementary to CMB.

Most of the cluster baryons are present as a diffuse gas, the Intra-Cluster Medium (ICM), which is
hot ($10^6 - 10^8$ K) and completely ionized. At these temperatures the ICM electrons Compton inverse
interact with CMB photons producing the thermal Sunyaev-Zel’dovich (SZ) effect (a distortion of
the CMB spectrum at the cluster position). In the last years survey dedicated instruments (Planck,
ACT, SPT) have finally been able to produce SZ selected catalogues containing several hundreds
of clusters. And in fact clusters and their SZ signal are becoming increasingly used to derive
cosmological constraints. However, cluster derived cosmology is limited by our ability to translate
cluster observables into mass estimates and tracers of the matter distribution. Then, a thorough
study of the scatter that the details of cluster morphology and astrophysics might introduce around
the average observable-to-mass behavior is mandatory.

With the final instrument NIKA2, to be permanently installed at the focus of the IRAM 30m
telescope, the NIKA collaboration aims to perform a systematic high angular resolution SZ follow-
up of a representative cluster sample of about 50 intermediate and high redshift clusters. NIKA, a
prototype of the final camera NIKA2, has already observed galaxy clusters via the SZ effect during
both technical time and Open Pools, providing high quality data for an high-z object, a
morphologically complex merging system and a Planck SZ-discovered cluster.
Vendredi 13 février 2015, 14h00
Salle de l'atelier, Paris
What do we learn from Planck polarized observations of Galactic dust ?
résumé :
The Planck satellite has completed the first whole sky map of dust polarization, providing unprecedented statistics and spectral information on the polarization properties of Galactic dust. I will present an overview of recently published Planck results on Galactic dust emission, both total and polarized.

These new data compare well with what is already known from visible extinction, polarized and unpolarized. Comparison of the statistical properties of dust polarization with MHD simulations show that the large scatter of the polarization fraction p observed at column density lower than 10^22 cm-2 is mainly driven by the fluctuations in the magnetic field orientation along the line of sight, rather than by variations of the grain alignment efficiency.

Five Planck polarized channels from 353GHz down to 70GHz has also allowed us to characterize the spectral dependence of dust emission. I will present how existing dust models deal with these new constraints on total and polarized emission, and comment on our ability to correctly infer the dust extinction and the associated gas mass from the observation of dust emission.

Vendredi 6 février 2015, 14h00
Salle de l'atelier, Paris
Testing gravity with cosmic voids
PUC, Chile
résumé :
Cosmic voids are defined as low density regions with 1/10 to 1/5 the average density of the universe. Voids usually span large volumes of equivalent radius of tens of Mpc. The low densities of such environments make them ideal probes for proposed modifications to gravity that are characterised by a fifth force that is expected to act in sparse environments. This is the case of f(R) models. In this talk I will summarize recent results on statistics of cosmic voids measured in General Relativity (GR) and f(R) simulations, and will show how one can tell apart these models using weak lensing.
Vendredi 30 janvier 2015, 14h00
Salle de l'atelier, Paris
The MESSIER satellite: unveiling galaxy formation
LERMA, Observatoire de Paris
résumé :
The MESSIER satellite has been designed to explore the extremely low surface brightness universe at UV and optical wavelengths.The two driving science cases target the mildly- and highly non-linear regimes of structure formation to test two key predictions of the LCDM scenario: (1) the detection of the putative large number of galaxy satellites, and (2) the identification of the filaments of the cosmic web. The satellite will drift scan the entire sky in 8 bands covering the 200-900 nm range to reach the unprecedented surface brightness levels of 32 mag/arcsec^2 in the optical and 37 mag/arcsec^2 in the UV. Many important secondary science cases will result as free by-products and will be discussed in some detail, such as the luminosity function of galaxies, the contribution and role of intracluster light, the cosmological background radiation at UV and optical wavelengths, the molecular hydrogen content of galaxies at z=0.25, the chemical enrichment of the interstellar medium through mass loss of red giant stars and the accurate measure of the BAO scale at z=0.7. It will provide the first space-based reference UV-optical photometric catalogue of the entire sky, and synergies with GAIA and EUCLID will also be discussed. The satellite will be submitted to the forthcoming call for a joint S-class mission by ESA and China, for a launch in 2021.
Vendredi 23 janvier 2015, 14h00
Salle de l'atelier, Paris
Galaxies That Shine: RHD Simulations of Radiation Feedback in Galactic Disks
Leiden Observatory
résumé :
Stellar radiation feedback is thought to be an important factor in regulating the evolution of galaxies. The radiation heats the galactic gas and pushes it via momentum transfer and as a result can play a role in stirring up the interstellar medium, suppressing star formation and perhaps even generating galactic winds. Radiation feedback is often included in numerical simulations of galaxy evolution. However, it is almost always modelled using pure hydrodynamics, with variously well motivated sub-grid recipes for the radiation, rather than from first principles with radiation-hydrodynamics (RHD), which are much more accurate, but complex and computationally expensive. The effects of those radiation feedback recipes are varied, and there is no consensus on how this feedback mechanism works in detail or whether it is actually important on the galactic scale.

We have implemented RHD in the widely used cosmological simulations code RAMSES. In order to gain an understanding of the effect of stellar radiation on galaxies, we use our implementation to evolve 20 parsec resolved galaxy disks in three dimensions. We disentangle the effects of photoionisation heating, direct pressure from photoionisation and pressure on dust particles from reprocessed infra-red (IR) radiation. We find that thermal pressure in the photoionised gas has an impact on the galaxies, similar to the inclusion of supernova feedback, in suppressing the formation of stars. Radiation pressure, whether direct from the ionising photons, or from reprocessed IR radiation, has little effect in our simulations, but this is to some degree due to the lack of resolution. The inclusion of stellar radiation has the effect of slightly decreasing gas outflows in winds, owing to reduced star formation, while the ratio of outflow rate versus star formation rate is increased.
Vendredi 9 janvier 2015, 14h00
Salle de l'atelier, Paris
La détection des modes B du CMB peut-elle se faire sans comprendre l’émission polarisée des poussières du milieu interstellaire ?
Jonathan AUMONT
IAS, Orsay
résumé :
De nombreuses expériences se sont aujourd’hui dédiées à la mesure des modes B primordiaux de la polarisation du Fond Diffus Cosmologique (CMB). Ce signal, dû à l’empreinte des ondes gravitationnelles primordiales, s’il était détecté serait en sus d’une sonde unique, la preuve ultime de l’existence de la phase d’Inflation survenue lors des premiers instants de l’Univers. Ces expériences, qui observent le ciel depuis le sol ou depuis des ballons stratosphériques, déploient des instruments à la sensibilité et à la maîtrise des effets systématiques sans cesse plus poussées. Mais, à l’instar de BICEP2 qui au mois de mars publia un signal sans doute trop vite attribué aux modes B primordiaux du CMB, il y a pour le moment une question à laquelle ces expériences ne peuvent pas répondre: quelle est l’amplitude des modes B de la polarisation des poussières galactiques, qui nous masquent le rayonnement de fond?

En effet, les grains de poussière interstellaire, de forme asymétrique, s’alignent au champ magnétique galactique pour produire une émission polarisée. Peu de choses étaient connues sur la statistique de cette émission sur de grandes régions du ciel avant que le satellite Planck, avec sa gamme de fréquence d’observation et sa couverture sans précédent à ces niveaux de sensibilité à la poussière, ne livre enfin ses réponses.

Ce sont ces premières réponses que je présenterai, en m’appuyant sur l’article que nous avons récemment rendu public (http://arxiv.org/abs/1409.5738). Je présenterai ce qui constitue la première mesure statistique de l’émission polarisée des poussières aux échelles spatiales d’intérêt pour la mesure des modes B du CMB. Je vous montrerai quelles propriétés générales ont pu en être tiré et comment nous avons utilisé ces informations pour démontrer qu’il n’existe pas de régions du ciel où l’empreinte des ondes gravitationnelles primordiales peut être mesurée en s’affranchissant du signal galactique. Enfin, je vous présenterai le niveau de poussière que nous avons mesuré directement dans la région observée par BICEP2 et qui invalide l’attribution hâtive de la totalité de leur signal au signal cosmologique."
Vendredi 19 décembre 2014, 14h00
Salle de l'atelier, Paris
The probability density function in molecular gas in the G333 and Vela C molecular clouds
UNSW, Sydney
résumé :
The probability density function (PDF) is a simple analytical tool for determining the hierarchical spatial structure of molecular clouds. It has been used frequently in recent years with dust continuum emission, such as that from the Herschel space telescope and ALMA. These dust column density PDFs universally show a log-normal distribution in low column density gas, characteristic of unbound turbulent gas, and a power-law tail at high column densities, indicating the presence of gravitationally bound gas. We have recently conducted a PDF analysis of the molecular gas in the G333 and Vela C giant molecular cloud complexes, using transitions of CO, HCN, HNC, HCO+ and N2H+.

The results show that CO and its isotopologues trace mostly the log-normal part of the PDF, while HCN and HCO+ trace both a log-normal part and a power law part to the distribution. On the other hand, HNC and N2H+ mostly trace only the power law tail. The difference between the PDFs of HCN and HNC is surprising, as is the similarity between HNC and the N2H+ PDFs. The most likely explanation for the similar distributions of HNC and N2H+ is that N2H+ is known to be enhanced in cool gas below 20K, where CO is depleted, while the reaction that forms HNC or HCN favours the former at similar low temperatures. The lack of evidence for a power law tail in 13CO and C18O, in conjunction for the results for the N2H+ PDF suggest that depletion of CO in the dense cores of these molecular clouds is significant. In conclusion, the PDF has proved to be a surprisingly useful tool for investigating not only the spatial distribution of molecular gas, but also the wide scale chemistry of molecular clouds.

Mercredi 10 décembre 2014, 14h00
Salle de l'atelier, Paris
jour de la semaine inaccoutumé
Cosmological simulations of structure formation
Benjamin L'Huillier
Korea Institute for Advanced Study, Seoul
résumé :
Cosmological simulations have become a powerful tool to describe the
non-linear regime of structure formation.

After reviewing the main aspects of cosmological simulations, I will
focus on the effects of the initial conditions on some statistics
obtained from cosmological $N$-body simulations, namely the density
power spectrum, the halo mass function, and the size distribution of
the large-scale structures (LSS), showing that second order Lagrangian
perturbation theory (2LPT) is needed to obtain 1% precision, and that
the starting redshift must be chosen with care.

I will then move on to galaxy evolution in a hierachical Universe.
I will describe galaxy mass assembly through merger of lower mass
companions and smooth gas accretion in multi-zoom hydrodynamical
simulations and show the importance of the smooth component.
Finally, I will focus on computing statistics of halo interactions
using the Horizon Run 4 cosmological $N$-body simulations, and studying
the effect of the environment on the interaction rates and its time
Vendredi 5 décembre 2014, 14h00
Salle de l'atelier, Paris
Galaxy and Star formation in Warm Dark Matter Cosmology
Nicola MENCI
INAF - Osservatorio Astronomico di Roma
résumé :
Building up an ''an initio'' theory of galaxy formation constitutes an major goal in astrophysics. However, models based on the Cold Dark Matter scenario encounter several fundamental and related problems at small galactic scales.

After reviewing such problems, I will focus on the cosmological models for galaxy formation based on Warm Dark Matter (WDM) model. I will present the effects of assuming a WDM power spectrum on the statistical properties of galaxies, including their star formation histories, their color properties, the abundance of substructures and the evolution of their luminosity functions. The impact of assuming a WDM spectrum on the evolution of the abundance and luminosity function on Active Galactic Nuclei will also be discussed.

Vendredi 28 novembre 2014, 14h00
Salle de l'atelier, Paris
In silico Miller experiments
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), UPMC
résumé :
The celebrated Miller experiments reported on the spontaneous formation
of amino acids from a mixture of simple molecules reacting under an
electric discharge, giving birth to the research field of prebiotic
chemistry. However, the chemical reactions involved in those experiments
have never been studied at the atomic level. Here we report on, to our
knowledge, the first ab initio computer simulations of Miller-like
experiments in the condensed phase. Our study, based on the recent
method of treatment of aqueous systems under electric fields and on
metadynamics analysis of chemical reactions, shows that glycine
spontaneously forms from mixtures of simple molecules once an electric
field is switched on and identifies formic acid and formamide as key
intermediate products of the early steps of the Miller reactions, and
the crucible of formation of complex biological molecules.
Vendredi 21 novembre 2014, 14h00
Salle de l'atelier, Paris
Scaling laws in Star & Structure formation: Density Models fitted to observational-Herschel and other- sets of data
Physics Department, University of Milan
résumé :
Stars form within molecular clouds, but progress on the study of these objects has been slow. Molecular clouds are among the coldest known objects in the Universe. Most of all we know today about their physical properties has been derived through radio spectroscopy of H2 surrogate molecules, but results obtained using these techniques, especially the estimate of column densities, are not always straightforward to interpret and are plagued by several poorly constrained effects.

Ultimately, our inability to accurately map the distribution of gas inside clouds has been a major impediment to understanding the star formation process. In the last few years, this situation has changed significantly. We can now use both dust extinction and emission to map robustly and at high resolution molecular clouds, and the high-resolution images provided by the Herschel satellite have shown the complex structure of molecular clouds and the pervasive presence of filaments. Additionally, we can use several multiwavelength observations ranging from the optical to the submillimeter to discover protostars and investigate the star-formation efficiency of different clouds.

Nearby molecular clouds represent our best chance to study the structure of molecular clouds and the early stages of star formation. In this talk I will present a coordinated study of the Gould Belt based on data from the 2MASS archive and the Spitzer, Herschel, and Planck missions. I will discuss the structure of these objects as revealed from large-scale extinction and emission maps, and present the scaling laws that have been found using these observations: the third Larson's law regarding the constancy of the surface density of clouds, the (local) Schmidt law for star formation, and the column-density probability distribution.
Vendredi 14 novembre 2014, 14h00
Salle de l'atelier, Paris
The instability of Astrophysics
Cornell University
résumé :
At some level, it is surprising that we seem to know so much about the Universeand so little about ourselves. What we learn about the Cosmos is based on telescopes, instrumentation, and data processing systems that we calibrate, test under a wide range of operating conditions, and continually monitor to ensure that the data obtained are reliable and fully understood. The process we understand least and calibrate most poorly is how we --- the astrophysical community --- determine the way the Cosmos functions.

We are convinced that in our everyday work we faithfully reveal the true nature ofthe Universe. But the history of our field shows that, time and again, we abruptly change course, with consequences that reverberate throughout astronomy. These instabilities can have long-lasting consequences in our field, because its various phenomena occur on such vastly differing scales as, for example, comets and supermassive black holes. The many different disciplines studying these varied phenomena are parts of a larger ecological network. Their effective symbiosis is required to advance our understanding of the Cosmos.

Can we find ways to manage this unstable system so it best serves its purposes?
Astrophysics cannot reject all instabilities, otherwise we’d never accept any
advances. But many instabilities are based on error, and we should seek to
control these as well as possible.

Is it possible to exert informed control where needed, or would such efforts discourage new insight and debate? We should at least recognize that stabilizing measures do exist in other walks of life to see whether adopting them might advance our understanding of the Cosmos. I am not aware of our community’s ever having studied these possibilities in depth but will try to show the forms they may take. Nations currently spend billions of dollars a year on astronomy, even as many people go hungry. Astronomers thus share a responsibility to spend these funds wisely.
Vendredi 7 novembre 2014, 14h00
Salle de l'atelier, Paris
Statistical analysis of the relative orientation between the magnetic field and dust filaments as seen by PLANCK
IAS, Orsay
résumé :
The role of the magnetic field in the formation of the filamentary structures seen in the interstellar medium is a debated topic. With the Planck polarization data, for the first time, we have the data needed to characterize statistically the structure of the Galactic magnetic field and its coupling to interstellar matter and turbulence at physical scales relevant to the formation of molecular clouds and interstellar filaments.

I will present a statistical study on the relative orientation between the matter structures traced by dust and the magnetic field in the ISM. I will discuss our results in light of theoretical and numerical scenarios on the formation of structures under the mutual action of turbulence, self-gravity and magnetic fields in the ISM.
Vendredi 24 octobre 2014, 14h00
Salle de l'atelier, Paris
SPICA: A far-infrared space observatory for the next decade
SRON, Pays-Bas
résumé :

The SPICA space telescope is a next generation far-infrared space observatory planned for launch in the late 2020s. Operating at wavelengths from 20-210um, it will bridge the wavelength gap between JWST and ALMA, and with its actively cooled primary mirror, provide unprecedented sensitivity in this wavelength range. In this presentation I will describe the current mission configuration and project status, as well as reviewing some of the key science objectives for this facility.

We should also make some kind of plan for what material it might be worth preparing in advance so that we can have some productive discussions regarding your disc deep search work.

Mercredi 15 octobre 2014, 14h00
Salle de l'atelier, Paris
jour exceptionnel
From Making Clouds to Making Outflows: Star Formation With the AstroBEAR AMR Multiphysics Code
University of Rochester
résumé :
Understanding star formation from the creation of molecular clouds to the feedback of stellar outflows on those clouds requires the development of state of the art Adaptive Mesh Refinement MHD multi-physics codes. In this talk I present two new studies using the AstroBEAR 2.0 code. In the first, I show MHD colliding flow simulations relevant to the formation of molecular clouds. Our models articulate the role of shear and field strength in the flows on the development of cloud properties such as mass, trapped magnetic flux and mixing. In the second study we present new simulations of Triggered Star Formation in which a shock sweeping over a magnetized cloud core triggers both collapse and the formation of an MHD outflow. I will also present a short overview of the code focusing on algorithmic challenges and strategies.
Vendredi 26 septembre 2014, 14h00
Salle de l'atelier, Paris
Reporté au 24 octobre
Bruce Sibthorpe
SRON, Leiden
Vendredi 20 juin 2014, 14h00
Salle de l'atelier, Paris
Radio Polarimetry and Cosmic Magnetism
Sydney Institute for Astronomy
résumé :
A remarkable discovery made by 20th century astronomers was that
the Universe is threaded with magnetic fields. Furthermore, these
magnetic fields typically do not have a random, tangled, morphology,
but are surprisingly organised and coherent. However, the processes
that create and then sustain this large-scale magnetism are not yet
understood. I will present innovative new observations of radio
polarisation and Faraday rotation, and will explain how these data
sets provide a unique view of magnetic fields in interstellar gas,
in the Milky Way, and in distant galaxies. These experiments pave
the way to the opening of the full magnetic Universe with the next
generation of radio telescopes, culminating in the Square Kilometre
Vendredi 13 juin 2014, 14h00
Salle de l'atelier, Paris
Formation des filaments dans le milieu interstellaire
résumé :
Si l'existence de filaments massifs dans le milieu interstellaire est connue depuis plusieurs décennies, le satellite Herschel a révélé leur omniprésence et a permis de quantifier leur distribution. En particulier, beaucoup d'entre eux semblent ne pas être autogravitants et de fait ne contiennent pas d'étoiles en formation. La question de l'origine de ces structures se pose donc. En effet, d'une part il est possible que ces filaments aient une influence sur la formation des étoiles qui se forment dans les nuages moléculaires. D'autre part et plus généralement, ils nous révèlent sans doute la nature des processus dynamiques à l'oeuvre dans ces nuages. A ce titre ils constituent un test pour la théorie des nuages moléculaires et la dynamique du milieu interstellaire en général.

Au cours de l'exposé je présenterai dans un premier temps les résultats observationnels récemment obtenus avec Herschel puis je décrirai plusieurs études ayant abordé spécifiquement l'origine de leur formation. Je distinguerai le cas des filaments autogravitants et celui des filaments plus diffus et proposerai que ces derniers aient pour origine la turbulence et le champ magnétique.
Vendredi 16 mai 2014, 14h00
Salle de l'atelier, Paris
"The HI-to-H2 Transition and HI Columns in Galaxy Star-Forming Regions"
Evelyne ROUEFF
Observatoire de Paris/LERMA
résumé :
I will present a new theoretical analytic approach describing the build up of atomic hydrogen (HI) gas columns in optically thick interstellar clouds, irradiated by far-ultraviolet photodissociating radiation fields [1]. We derive analytic expressions for the total HI column densities for (1D) planar slabs, for beamed or isotropic fields, from the weak-to-strong-field limits, for gradual or sharp atomic transitions, and for arbitrary metallicity. We make the distinction between "HI-dust" and "H2-dust" opacity. Our expressions may be used to evaluate the HI column densities as functions of the radiation field intensity, the hydrogen gas density, the metallicity dependent H2 formation rate and far UV dust-grain absorption cross section. We validate our analytic formulae with the Meudon PDR code. We apply our theory to compute H2 mass fractions and star-formation thresholds for individual clouds in self-regulated galaxy disks, for a wide range of metallicities. Those formulae may be
incorporated into hydrodynamics simulation for galaxy evolution.

[1] Sternberg A., Le Petit F., Roueff E. and Le Bourlot J., 2014, Astrophysical Journal Supplement, in press
Vendredi 11 avril 2014, 14h00
Salle de l'atelier, Paris
Gas and Stellar Properties of Galaxies at z>=2 in Cosmological Hydrodynamical Simulations
Paramita BARAI
INAF, Trieste
résumé :
The large-scale-structures of galaxies, clusters and filaments are believed to have formed from tiny density fluctuations in the early universe. Numerical simulations have been proven to be a powerful tool to follow the non-linear evolution of matter over the Hubble time. I will report our ongoing development and performance of cosmological hydrodynamical simulations, and subsequent analysis of simulated galaxy and circumgalactic medium (CGM) properties at high-z. Feedback from star formation (SF) and supernova (SN) explosions as mass / energy ejection play crucial roles in galaxy-formation. We numerically implement novel baryonic feedback models in the 3D TreePM-SPH code GADGET-3, using which we simulate cosmological volumes, also including radiative cooling, SF, and chemical enrichment. The impact of different feedback mechanisms on the global properties at z>=2 are analyzed, and confronted with observations, where available. Some of the properties are: stellar mass function; gas fraction, stellar fraction, SF rate, gas outflow velocity and mass loading in galaxies; physical state (density, temperature) and metal enrichment of the CGM. Results from such studies will
be presented.
Vendredi 14 mars 2014, 14h00
Salle de l'atelier, Paris
Discovering the sky polarization with Planck
résumé :
The Planck satellite has recently completed the first whole sky map of polarization at sub-mm and mm wavelengths. This is an immense step forward from earlier polarization observations of the CMB and Galactic dust. The data is revealing a new sky we have started to explore. Our paramount goal is the search for a signature of gravity waves generated in the early expansion of the universe in the CMB polarization. However, dust polarization has a much larger amplitude than the expected signal. The signature of cosmic inflation will not be detected, unless contamination associated with the dusty and magnetized interstellar medium in the Galaxy is removed with the required accuracy and confidence. This requirement couples the CMB research to Galactic astrophysics. With the Planck survey, for the first time, we have the data needed to characterize the structure of the Galactic magnetic field and its coupling with interstellar matter and turbulence, in the diffuse interstellar medium and molecular clouds.

Here our work aims at revealing the role the magnetic field plays in the formation of the filamentary structure of interstellar matter. Our analysis of the data also involves the characterization of the polarization properties of dust, and the alignment of grains with the magnetic field. I will introduce the science questions we are investigating and present our first results on the CMB and dust polarization.
Vendredi 7 mars 2014, 14h00
Salle de l'atelier, Paris
Le regard de Fermi sur les rayons gamma et cosmiques
Isabelle GRENIER
AIM Université Paris Diderot & CEA Saclay
résumé :
Le télescope gamma spatial Fermi observe le ciel depuis l'été 2008.
Il a détecté plus de 2800 sources émettrices de rayons gamma (0.1-100 GeV)
au sein desquelles agissent de puissants accélérateurs de particules
(éruptions de trous noirs supermassifs, sursauts gamma, pulsars,
sources binaires, novae, restes de supernova, éruptions solaires, ~).
Je brosserai un rapide panorama des principales questions soulevées par
les observations de Fermi sur ces phénomènes d~accélération encore
mal cernés, avant de me tourner vers un acteur beaucoup plus calme
du ciel gamma mais tout aussi déroutant: les rayons cosmiques.
Leur émission gamma se révèle également d'une grande richesse, en divers
lieux de la Voie Lactée mais aussi dans quelques galaxies voisines
et dans une poignée de galaxies à flambées d'étoiles. Elle fournit
un moyen de sonder le flux de rayons cosmiques et la quantité de gaz total
qu'ils traversent dans différents contextes. On a ainsi trouvé un taux
inexpliqué de rayons cosmiques dans les régions externes de la Galaxie.
L'émission gamma a également révélé la présence de tous jeunes rayons
cosmiques, à leur source dans des restes de supernova ou bien piégés
dans un cocon de jeunes étoiles dont l'existence bouscule nos idées
sur la propagation ultérieure des particules, une fois échappées de
leur source. Un florilège de résultats marquants sera présenté
et mis en perspective.
Vendredi 21 février 2014, 14h00
Salle de l'atelier, Paris
Analysis of commonly used SFR indicators for a sample of Far-IR detected galaxies
Departamento de Astrofísica, Universidad Complutense de Madrid (UCM)
résumé :
One of the main aspects to understand galaxy formation and evolution
focuses on the mass assembly of galaxies at different epochs.There are
three galaxy properties which are fundamental when studying the
formation and mass assembly of the galaxies and that are strongly
interrelated to each other: the galaxy stellar mass (M), the metallicity
(Z) and the star formation rate (SFR). The stellar masses are usually
derived through SED-fitting without significant errors. For the
metallicity derivation it is strictly necessary to have accurate
emission lines values. To derive the SFR, however, there are many
different recipes focusing on the emission at different wavelenghts. As
the SFR is a fundamental parameter in galaxy evolution, it is essential
to compare the different SFR indicators and to understand which galaxy
properties have a more important impact on their agreement/disagreement.
We will review results on the comparison of SFR from Halpha, UV or IR
luminosities using samples of galaxies with Far-IR detection from the
PACS Herschel data (100-160 microns). SFR(IR) has the advantage of not being
affected by dust extinction. For the optical SFRs we use different dust
extinction recipes (UV slope, Halpha/Hbeta ratio and LIR /LUV ratio). We find a
very good agreement between the SFRs with smaller dispersions than
typical uncertainties. We also study the effect of mass and metallicity
in this comparison.
Vendredi 7 février 2014, 14h00
Salle de l'atelier, Paris
Planet formation and vortices
Héloïse MEHEUT
résumé :
One of the most challenging step in planet formation theory is the one explaining the formation of planetesimals of kilometre size. Vortices on Earth are known to concentrate solid particle in the eye of the vortex, and a similar process is proposed to concentrate solids in protoplanetary disks and participate to planet formation. This vortex scenario is in vogue since the asymmetries observed in protoplanetary disks by the Atacama Large Millimeter Array (ALMA) are proposed to be observational evidences of the presence of Rossby vortices in the disks.

In this talk, I will introduce the vortex scenario for planetesimals formation and discuss the formation of large scale vortices in protoplanetary disks by the Rossby wave instability. I will present an analytical study and global numerical simulations of this instability leading to the formation of Rossby vortices. Then I will detail the 3D structure of these vortices and its influence on solid concentration. I will conclude by showing how ALMA observations could be compared with this numerical study of solid concentration in vortices.
Vendredi 31 janvier 2014, 14h00
Salle de l'atelier, Paris
From the First Stars to Galaxy Clusters: Understanding Structure
résumé :
Computational cosmology offers an extremely powerful tool to investigate
how stars, galaxies, and clusters came about. Today, our quest has
advanced far beyond the large-scale structure that is mainly constituted
by dark matter. Baryonic gas and its complex dynamics as well as
feedback from stars and supermassive black holes contribute
to an extremeley rich picture of structure formation, covering a vast
range of scales. I will discuss some of the involved processes by
reporting on numerical simulations of (1) primordial atomic cooling
halos, (2) feedback processes in disk galaxies, and (3) the formation of
clusters. Moreover, I will touch upon new approaches to tackle the
problem of the generation of cosmic magnetic fields. One of the big
challenges simulators are facing now is to link their numerical data to
observations. I will conclude with commenting on our current perspective
Vendredi 10 janvier 2014, 14h00
Salle de l'atelier, Paris
Stellar feedback in semi-analytic models of galaxy formation.
Andrea Cattaneo
Observatoire de Marseille
résumé :
The formation of galaxies begins with the gravitational instability of primordial density fluctuations in the dominant dark-matter component. Galaxies are formed by gas that radiates, sinks into the gravitational potential of dark matter haloes and fragments into stars, but star formation is not the end of the galaxy formation process because stars can feed matter, metals, energy and momentum back into the interstellar and the intergalactic medium in a way that contributes to shape the global properties of galaxies (stellar feedback).

This talk, I shall review the physical processes that are responsible for stellar feedback, their effects on a galactic scale and the motivation for including stellar feedback feedback in galaxy formation models. I shall discuss how stellar feedback is implemented in semi-analytic models and compare their results with the insight from hydrodynamic simulations. I shall conclude with a summary of the results that may be considered and of the main outstanding problems.
Mardi 17 décembre 2013, 14h00
Salle de l'atelier, Paris
Attention jour exceptionnel
Chemodynamical simulations with AGN feedback
University of Hertfordshire Science & Technology, UK
résumé :
The formation and evolutionary histories of galaxies are imprinted in stellar populations, namely, in elemental abundance ratios. I am simulating chemodynamical evolution of galaxies using my hydrodynamical code that includes star formation and chemical enrichment from core-collapse and Type Ia supernovae, and AGB stars. Then we succeeded in reproducing various observations such as the elemental abundance ratios in Galactic bulge and disk radial gradients, and mass-metallicity relations of galaxies. However, a problem that star formation is not quenched in massive galaxies remains. To solve this, we recently included AGN feedback into our cosmological simulations.I'll also show some of new results with the results AGN feedback.
Vendredi 6 décembre 2013, 14h00
Salle de l'atelier, Paris
Cosmic Vortices in Hot Stars and Cool Disks
Columbia University, N-Y
résumé :
The radiation that permits us to observe cosmic bodies also
plays a role in their structures and evolution. While the thermal
aspects of the radiation are familiar, at least qualitatively,
the dynamical effects of the radiation are perhaps less so.
These dynamical effects are becoming quite important in current
astrophysical studies. This subject, which I have provisionally been
calling photofluiddynamics after some discussion has a number
of applications to cosmic objects. The most massive stars known are
very hot and are the sites of vigorous fluid dynamical activity.
The processes involved are of interest, not only in themselves, but
also in the way they affect the observed features of the hottest
stars by forming coherent vortices and magnetic flux tubes.
Similar structures in accretion disks, particularly in protoplanetary
systems, may arise and play important roles in the evolution of those
objects. I shall consider only cool disks that are probably like
the primitive solar nebula in which vortices may participate in the
formation of planets.
Vendredi 29 novembre 2013, 14h00
Salle de l'atelier, Paris
Eléonore SAURY
résumé :
One of the main current questions in Astrophysics is the understanding of the star formation process, directly related to the processes involved in the cooling and the condensation of the gas yielding to intricate filamentary structures of molecular clouds. Magnetic field, thermal instability and turbulence are playing dominant roles in this complex dynamics.

The work presented here is focused on the evolution of the atomic and diffuse interstellar medium that provides the initial conditions to the formation of molecular clouds. To understand the roles of thermal instability and turbulence and describe the warm neutral medium (WNM) to Cold neutral medium (CNM) transition, we use hydrodynamical numerical simulations of thermally bistable HI from which we compute synthetic 21 cm spectra that are compared to observations of high Galactic latitudes clouds.

With WNM initial conditions and stirring in of turbulent motions at large scales, the simulations reproduce basic observational statistics of the diffuse ISM. But some kind of compression is needed to create CNM, either with a majority of compressible modes of the injected turbulent velocity field or with a higher initial density than the WNM fiducial density. The pressure range obtained matches what is measured from atomic absorption lines and the mass in each thermal phase is compatible with 21 cm observations, including 1/3 of the mass in the thermally unstable regime. Moreover, the shapes of the synthetic 21cm spectra are morphologically very similar to the observed ones.

All these results were obtained with a sonic turbulence. They imply that, given the properties of the heating and cooling mecanisms, the formation of cold and dense structure in the diffuse ISM occurs naturally in the presence of sonic large scale turbulent motions and compression.
Mardi 26 novembre 2013, 14h00
Salle de l'atelier, Paris
exceptionnellement mardi 14h
Development of THz Superconducting Mixers and Detectors for DATE 5
Purple Mt Observatory, China
résumé :
The terahertz (THz) regime is the frequency window to be fully explored in astronomy.Dome A in Antarctic, with an altitude of 4093 m and temperature below -80 Celsius degree in winter, is regarded as the best site on earth for astronomical observations in this frequency regime. Currently, China is proposing to build an observatory there, in which Dome A 5m Terahertz Telescope (DATE5) is one of two major telescopes. This talk will mainly present the development of THz superconducting mixers and detectors for DATE 5.
Vendredi 11 octobre 2013, 14h00
Salle de l'atelier, Paris
Analyzing Terrestrial Planet Finder Capabilities and Extrasolar Giant Planet Spectroscopy with Artificial Neural Networks
Ted von HIPPEL
Embry-Riddle Aeronautical University
résumé :
Artificial Neural Networks (ANNs) were developed to model biological
information processing systems. Like biological neural networks, ANNs
can learn and are often considered to be a form of artificial intelligence.
Subsequent work with ANNs demonstrated that they are excellent at
classifying noisy data, interpolating in high-order parameter space, and
yield classifications that are equivalent to Bayesian probabilities. I
will present an ANN study of instrumental trade-offs for possible
Terrestrial Planet Finder instruments and a new ANN approach to fitting
exoplanet spectroscopy to model planetary atmospheres.

Vendredi 27 septembre 2013, 14h00
Salle de l'atelier, Paris
Planck et les grandes structures
Hervé DOLE
résumé :
Le satellite Planck a observé la totalité du ciel
pendant près de 2 ans et demi, dans le but de
mesurer le rayonnement fossile de l'Univers, i.e.
le fond diffus cosmologique, avec une
précision sans précédent. Nous venons de publier
les données des 15 premiers mois de mission (le reste
le sera dans un an).

Après une mise en contexte des grandes questions en
cosmologie et astrophysique (inflation, formation des
grandes structures, matière noire, énergie noire notamment),
je propose une vue d'ensemble synthétique des principaux
résultats cosmologiques concernant les
paramètres cosmologiques, l'inflation, la formation
des structures.

J'insisterai ensuite sur la richesse du catalogue de sources
rendu public (galaxies et amas de galaxies), et aborderai la
question de la recherche des premiers amas de galaxies et de
formation stellaire à grand redshift.

J'esquisserai pour finir des perspectives pour l'année
prochaine (en particulier les données de polarisation)
concernant l'analyse des données Planck et les synergies
futures avec le satellite Euclid.
Vendredi 20 septembre 2013, 14h00
Salle de l'atelier, Paris
The Cosmic Spectral Evolution of Starbursts and AGN
Australian National University, Canberra
résumé :
We use the chemical evolution predictions of cosmological hydrodynamic
simulations with our latest theoretical stellar population synthesis,
photoionization and shock models to predict the optical strong line
evolution of ensembles of galaxies from $z=3$ to the present day.
We show that star forming galaxies at high redshift ($z>1.5$) are
consistent with a model in which the ISM conditions are more extreme
at high redshift than seen in the global spectra of local galaxies.
We speculate that global spectra of our high redshift galaxies may
be dominated by HII regions similar to the extreme clumpy, dense star-forming
complexes in the Antennae and M82. We show that rest-frame optical spectra
of AGN may probe metallicity gradients as a function of redshift. We show
that at high redshift, galactic wind shocks are clearly separated from AGN
in line ratio space. Instead, shocks from galactic winds mimic high metallicity
starburst galaxies. We discuss our models in the context of our current
understanding of the chemical evolution of galaxies and in the context
of future large near-infrared spectroscopic surveys.
Vendredi 13 septembre 2013, 14h00
Salle de l'atelier, Paris
seminaire du 28 juillet reprogrammé le 6 septembre
0 < z < 4 redshift evolution of the total UV+IR SFRD and dust attenuation
Laboratoire d'Astrophysique de Marseille
résumé :
Using new and homogeneous Luminosity Functions (LFs) in FUV (VVDS) and in FIR Herschel/PEP and Herschel/HerMES, we study the evolution of the dust attenuation with redshift. With this information in hand, we are able to estimate the redshift evolution of the total (FUV + FIR) star formation rate density (SFRD_TOT). By integrating SFRD_TOT, we follow the building of mass and analyze the redshift evolution of the stellar mass density (SMD). This paper aims at providing a complete view of star formation from the local universe to z~4 and, using assumptions on earlier star formation history, compares this evolution to what was known before in an attempt to draw an homogeneous picture of the global evolution of star formation in galaxies.
The main conclusions of this paper are 1) the dust attenuation A_FUV is found to increase from z=0 to z~1.2 and then starts to decrease up to our last data point at z=3.6. 2) The estimated SFRD confirms published results up to z ~ 2. At z>2, we observe a plateau or a small increase up to z~3 and then a likely decrease up to z=3.6. 3) the peak of A_FUV is delayed with respect to the plateau of SFRD_TOT. A likely origin might be found in the evolution of the bright ends of the FUV and FIR LFs. 4) Using assumptions (namely exponential rise and linear rise with time) for the evolution of the star formation density from z=3.6 to z_form=10, we integrate SFRD_TOT and find a good agreement with the published SMDs up to almost z_form.
Finally, I will present a new project dedicated to observing the first galaxies at z > 8 - 15 : WISH in collaboration with JAXA (Japan) and CNES.
Vendredi 19 juillet 2013, 14h00
Salle de l'atelier, Paris
The Dynamics of Galaxy Pairs in a Cosmological Setting
University of Victoria
résumé :
Galaxy pairs provide a unique view of the interaction sequence experienced by
merging galaxies. Observationally, interactions have a dramatic influence on
galaxies, even during the earliest stages. Theoretically, a large industry of
numerical merger simulations has developed. Unfortunately, the latter depend on
the assumption that interacting galaxies evolve in isolation. A central goal of
this work is to investigate the validity of this assumption. Using the
Millennium Simulation, we built a large catalogue of simulated galaxy pairs.
For each pair, we searched for a more massive 'third' galaxy in the vicinity.
A comparison of the binding-energy of the pair to the binding energy to the
third galaxy allows us to rank pairs in terms of their probability of merging.
The results are as follows: (a) 12% of the pairs are inevitable mergers in
isolation; (b) 37% are likely mergers, with minimal influence of a third
massive galaxy in the vicinity; (c) 20% will most likely interact, but not
merge because the third galaxy will split them apart; and (d) 30% are chance
pairs orbiting a third massive galaxy, and will never merge. This work
demonstrates the importance of connecting galaxy pairs to the rest of the
Universe, and provides guidance to both observers and simulators on how
realistic it is to treat merging galaxies in isolation. Lastly, I will discuss
ongoing work based on binary merger simulations. These two complementary
methods (semi-analytics and hydro-simulations) will help us bridge the gap
between galactic and cosmological scales, and enrich our understanding of the
physical processes governing the interaction sequence.
Mercredi 3 juillet 2013, 14h00
Salle de l'atelier, Paris
Outflows from galaxies and AGN
Mahavir SHARMA
Raman Research Institute, Bangalore
résumé :
Galactic outflows are multiphase hydrodynamic phenomena observed in various wavelengths, and are thought to be driven by supernovae and AGN activity. They are important as a feedback process for galaxy formation and evolution. In this talk, I will discuss new results from our study of these multiphase outflows from hydrodynamical and cosmological perspective. We find that supernovae driven outflows can escape only from low mass galaxies and their speeds rarely exceed 1000 km/s. On the other hand, AGN momentum injection can drive outflows from massive galaxies with speeds exceeding this limit. This result provides a useful diagnostic, and is also supported by observations. We also derive the relation between stellar and halo mass for galaxies undergoing outflows, and compare with observations. We also study the interaction of a free wind with the hot halo gas, and quantify the conditions for the escape of free wind from galaxies. We find that the extraplanar cold/warm clouds can be formed in these interaction zones via fluid instabilities. We have developed a radiation and ram pressure driven outflow model to study the cold/warm phase of outflows and show different regimes of SFR and galactic mass in which ram and/or radiation pressure dominates. Recently we have also carried out an observational study to explore the possibility that MgII absorbers in quasar sightlines are associated with quasar radiation driven outflows.
Vendredi 21 juin 2013, 14h00
Salle de l'atelier, Paris
The role of feedback in galaxy formation models
Institute for Theoretical Physics, Zurich
résumé :
I will present some recent results obtained using cosmological simulations of
galaxy formation. I will discuss feedback processes across different scales,
advocating that they might play a dominant role in shaping spiral and elliptical
galaxies. I will discuss current phenomenological models for feedback, and
outline future work to design more physical models of supernovae, radiative and
active galactic nuclei feedback.

Vendredi 14 juin 2013, 14h00
Salle de l'atelier, Paris
Galaxy profiles from observations and dark matter properties
résumé :
We show how to explain the observed kinematics, the weak and strong lensing signals and the X-ray emissions of Galaxies, by means of a non-luminous component embedding the various baryonic components . Understanding the detailed properties of this component labelled Dark Matter,(DM) is one of the most pressing issues of Cosmology. We point out that the observed relationship between the galaxy radial coordinate r, and the dark halo contribution to the mass at r , leads to the framework of the Universal Rotation Curve (URC) paradigm. We find, in general agreement with many other studies, that the universal distribution of Dark Matter in galaxies emerging from data is in evident inconsistency with predictions in the LambdaCold Dark Matter (LambdaCDM) scenario, also when cosidering the feedback of baryons on the DM halos. We show that observations define a very clear scenario that points to galaxies being macroscopic quantum systems in the Warm Dark Matter scenario.
Vendredi 17 mai 2013, 14h00
!!! Salle DANJON !!!, Paris
Salle inhabituelle
Heating and Cooling of the ISM in NGC 1097
Observatoire de Paris/LERMA
résumé :
NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear
starburst ring, a strong large-scale bar and an active nucleus. One of the
goals of KINGFISH Herschel Open Time program is to study general trends of
the ISM heating and cooling in nearby galaxies. I present a detailed study
of the spatial variation of the far infrared (FIR) [CII]158um and [OI]63um
lines and mid-infrared H2 emission lines as tracers of gas cooling, and of
the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the
photoelectric heating. The photoelectric gas heating efficiency
([CII]158um+[OI]63um)/PAH in the ring is estimated as ~ 50% lower than in
the spiral arms. The average 11.3/7.7um PAH ratio is also lower in the
ring, which may suggest a larger fraction of ionized PAHs, but no clear
correlation with [CII]158um/PAH(5.5-14um) is found. PAHs in the ring are
responsible for a factor of two more [CII]158um and [OI]um emission per
unit mass than PAHs in the Enuc S, which can be an alternative explanation
for the differences in gas heating efficiency. Much of the the H2 emission
in the starburst ring could come from warm regions in the diffuse ISM that
are heated by turbulent dissipation or shocks. I also present results for
other galaxies in the KINGFISH sample, focusing on starburst rings and
other resolved regions of enhanced star-formation.
Vendredi 19 avril 2013, 14h00
Salle de l'atelier, Paris
Un aperçu des résultats de Planck en 2013
Jean-Michel LAMARRE
Observatoire de Paris/LERMA
résumé :
29 articles ont été soumis et rendus publics fin mars sous l'appellation commune
"Planck 2013 Results:...". Ils constituent la première publication des résultats
cosmologiques de Planck, s'appuyant sur les 15 premiers mois de données et l'état
actuel de leur traitement. Simultanément, les données traitées et calibrées sont
rendues publiques. Les résultats concernant la polarisation seront disponibles
ultérieurement. Ces articles décrivent les données, leur traitement et leur
interprétation. La mesure du fond cosmique micro-ondes montre un accord
avec le modèle LambdaCDM avec une précision sans précédent. D'autres résultats
remarquables illustrent l'intérêt de Planck pour les astrophysiciens. Je
m'efforcerai de donner à la fois un aperçu des résultats principaux et un point
de vue personnel sur le déroulement de ce projet hors normes à plus d'un titre.
Vendredi 5 avril 2013, 14h00
Salle de l'atelier, Paris
From planetary to stellar dynamos: what can we learn about the magnetism of M dwarfs?
Max-Planck-Institut für Sonnensystemforschung
résumé :
The magnetic fields of planets and rapidly rotating stars are maintained by
convection-driven dynamos operating in their interiors. Scaling laws derived
from geodynamo-like models successfully predict the magnetic field strength of
a wide range of astrophysical objects from Earth and Jupiter to some
rapidly-rotating stars. This emphasises the similarities between the dynamo
mechanisms at work in planets and active M dwarfs.

Recent spectropolarimetric observations of M stars show a broad variety of
large-scale magnetic fields encompassing dipole-dominated and multipolar
geometries. Combining global-scale numerical dynamo models and observational
results, we want to better understand the similarities of dynamos in planets
and low-mass stars. To study the physical mechanisms that control the magnetic
field morphology in these objects, we have explored the influence of rotation
rate, convective vigor and density stratification on magnetic field properties
in anelastic dynamo models.

In such models, the relative importance of inertia in the force balance -
quantified by the local Rossby number - is thought to have a strong impact on
the magnetic field geometry. The observed transition between dipole-dominated
and multipolar large-scale dynamos in early to mid M dwarfs is therefore
tentatively attributed to a Rossby number threshold. We interpret late M
dwarfs magnetism to be the consequence of a dynamo bistability occurring at
low Rossby number, and predict different amplitudes of differential rotation
on these two dynamo branches. Th
Vendredi 15 mars 2013, 14h00
Salle de l'atelier, Paris
Physics and chemistry of UV illuminated neutral gas: The Horsehead case.
résumé :
The Horsehead mane is a particularly interesting case for the study of photodissociation regions because the transition from the diffuse, hot and ionised gas to the dense, cold and shielded gas is sharp. The PDR geometry is simple (viewed edge-on) and the density profile across the PDR is well constrained. The combination of small distance to Earth (at 400 pc, 1" corresponds to 0.002 pc), low illumination (χ= 60) and high density (nH ~ 105 cm-3) implies that all the interesting physical and chemical processes can be probed in a field-of-view of less than 50" (with typical spatial scales ranging between 1 and 10"). The rsehead PDR is a good source to benchmark the physics and chemistry of UV illuminated neutral gas.
In this talk, I will summarize some results from the Horsehead WHISPER, a sensitive, high spectral resolution, full 1, 2 and 3mm spectral survey of 2 positions in the Horsehead Nebula obtained at the IRAM 30m telescope. This will include 1) H2CO, CH3CO photo-desorption 2) (iso)-nitriles in UV illuminated gas 2) the 2nd detection of CF+ (hyperfine structure and fluorine abundance) 3) the first detection of the new hydrocarbon cation C3H+.
Vendredi 1 mars 2013, 14h00
Salle de l'atelier, Paris
Presentation of IRIS, a Generic 3D Radiative Transfer Code for Spectral Diagnostics. Application to Radiative Shocks"
Laurent IBGUI
Observatoire de Paris/LERMA
résumé :
IRIS is a new generic 3D radiative transfer code for the prediction of high-resolution synthetic spectra (Ibgui et al. 2013). IRIS solves the monochromatic 3D radiative transfer equation. This tool provides the capability of relating theoretical Magneto Hydrodynamics or Radiation Hydrodynamics (RMHD) models to the spectral emissions of the studied objects or structures for further comparisons with observations.

I will outline the key features of IRIS. In particular, the code can handle periodic media. It also takes into account the velocity gradient effect for arbitrary velocity fields. I will introduce its future extensions that will allow the treatment of scattering and non-LTE situations.
IRIS has post-processed a 3D RHD model of a radiative shock calculated by the 3D code HERACLES. I will discuss the hydrodynamic and radiative structure of the shock. I will comment on the results of a comparison of the radiation moments calculated by the grey M1 model of HERACLES and by the exact method of IRIS. Further, I will present theoretical spectra that emerge from radiative shocks, and discuss their angular and spectral distributions.
Vendredi 22 février 2013, 14h00
Salle de l'atelier, Paris
Warm Dark Matter Galaxies in agreement with Observations: Formation, Evolution and Supermassive black holes. New results.
CNRS LERMA-Observatoire de Paris
résumé :
Dark matter (DM) is the dominant component of galaxies. Warm dark matter (WDM, DM particles of mass in the keV scale) is able to solve naturally the serious drawbacks of cold dark matter (CDM) at small and galactic scales, keeping all its successes at large and cosmological scales. Namely:
(i) WDM produces the correct abundance of substructures at redshift zero and higher redshifts.
(ii) Quantum mechanics turns to play a crucial role in small scale WDM galaxy structure (kpc scales and below).
(iii) Compact dwarf galaxies turn to be natural quantum macroscopic objects, supported against gravity by the fermionic WDM quantum pressure, while the large galaxies appear naturally in the dilute and classical regime.
(iv) The Thomas-Fermi semiclassical approach of atomic physics implemented here for fermionic WDM galaxies yields the main physical galaxy magnitudes: mass, core radius, phase-space density, velocity dispersion, fully consistent with observations for all types of galaxies from the compact dwarfs to the large galaxies, spirals, ellipticals . In particular, quantum fermionic WDM provides galaxy cores and their sizes in the right observed scales. The theory of galaxy structures, formation and evolution turns to be considerably clarified in WDM and placed in the framework of physics and cosmology. WDM galaxies are the analogous of stars as treated by Chandrasekhar, Landau, Bethe, ......
Vendredi 8 février 2013, 14h00
Salle (à venir), Paris
Effects of the stochasticity of galaxy angular momentum growth on star formation
Pontificia Universidad de Chile, Santiago
résumé :
Dark matter (DM) haloes, the cradles where galaxies form and evolve, acquire angular momentum in a stochastic way which results in a final non-zero DM adimensional spin parameter. We will see how this process translates into the ability of the galaxy, living in this DM halo, to form stars. To do this we will first measure this stochasticity from the Millennium II simulation, and then see how it affects the properties of the disc of baryons that forms in the center of a DM halo. We will see that the smooth infall of matter comes in chunks with almost, but not quite, random angular momenta, which affects the ability of a disc to form stars and also the frequency of instabilities in the disc leading to the formation of bars and pseudo-bulges.
Vendredi 25 janvier 2013, 14h00
Salle de l'atelier, Paris
Starburst and old populations in distant z>4 radio galaxies with the code Pégase.3
Paris-Sud & IAP
résumé :
The spectral synthesis of 2 powerful distant (z=4) radio galaxies
are computed with the new code Pégase.3, predicting stellar, gaz, metal
masses and the coherent dust emission, covering the UV-IR-submm range
(Rocca-Volmerange et al. 2013).
Two young and old stellar components (starburst and elliptical galaxies)
are identified as well as a faint active nucleus. Masses and ages
provide strong constraints on the physics of galaxy evolution likely
driven by massive starbursts from the earliest epochs of the Universe.
Consequences on theories are discussed
Vendredi 18 janvier 2013, 14h00
Salle de l'atelier, Paris
The molecular emission from Supernova Remnants
Antoine Gusdorf
résumé :
Supernova Remnants (SNRs) play a vital part in the interstellar medium, where they re-distribute large amounts of energy, and probably constitute the primary sites for accelerating galactic cosmic rays. In my talk, I will present recent observations, mostly from the APEX, SOFIA (CO), and Spitzer (H2) telescopes, of various SNRs, all detected at TeV energies by the Fermi telescope. I will show that such observations are an efficient way to improve our knowledge of the physical and chemical conditions prevailing in SNR environments. I will also show how comparisons with shock models constitute a valuable tool to constrain both the shock characteristics and pre-shock conditions, leading to accurate estimates of shocked gas masses and related energetics. During this talk, I will focus on the F knot of the SNR W28, and present how our various IC443 observation programs will contribute to make this object a reliable template for the study of Galactic SNRs.

Vendredi 7 décembre 2012, 14h00
Salle de l'atelier, Paris
Turbulent convection in stellar interiors: a numerical approach using time-implicit simulations
University of Exeter, UK
résumé :
Convection is a key process in stellar interiors. It is characterized by large
Reynolds numbers, implying a highly turbulent regime. I will show how
multidimensional hydrodynamical simulations allow us to get a physical insight
of this complex phenonemon. The numerical results are obtained with a novel
hydrodynamical code, MUSIC, which is based on time-implicit methods. I will
present the general framework and discuss the advantages of implicit methods in
tackling stellar hydrodynamical problems.
Vendredi 23 novembre 2012, 14h00
Salle de l'atelier, Paris
Les carbones amorphes hydrogénés : observations, synthèse et caractérisation en laboratoire de poussières interstellaires.
GSFC, Greenbelt
résumé :
La poussière est une composante majeure des galaxies. Son étude au travers d’expériences de laboratoire, associées aux observations, permet de déduire la composition et l’évolution des différents types de grains du cycle de la matière dans une galaxie.
Une partie importante de la poussière interstellaire est constituée de carbones amorphes hydrogénés (a-C:H ou HAC). Ces grains hydrocarbonés sont détectés par leurs signatures spectrales infrarouges, largement observées dans le milieu interstellaire diffus de la Voie Lactée ainsi que dans de nombreuses autres galaxies. Le fait qu’ils ne soient, par contre, pas détectés dans les nuages interstellaires denses pose la question de leur évolution induite par les processus d’altération astrophysiques.
Je présenterai mes travaux sur l’étude de ces grains interstellaires. Les récents résultats, obtenus à la fois au travers de nouvelles observations, ainsi que grâce à la synthèse et la caractérisation d’analogues de laboratoire, permettent notamment de mieux connaitre l’émission visible de ces poussières carbonées ainsi que leur évolution due aux rayons cosmiques.
Vendredi 9 novembre 2012, 14h00
Salle de l'atelier, Paris
Molecular line observations of protoplanetary disks
Academia Sinica, IAA, Taiwan
résumé :
Understanding the structure and evolution of disks surrounding young low-mass stars is one of the key issues to study the process of planet formation. Nevertheless the overall properties of those disks are not yet well constrained by observations. Observations of molecular lines is a useful tool to constrain the disks physical structure, as different molecules sample different physical conditions. Beside the abundant CO, several other molecules have been detected in the outer part of the disks in the millimeter domain (e.g. HCO+, H2CO, CS, HCN, CN...). In this talk I will present recent results obtained with both single dish and interferometer, and I will confront them to models of
protoplanetary disks, in particular to the layered structure that is predicted by all chemical model so far.
Vendredi 12 octobre 2012, 14h00
Salle de l'atelier, Paris
Recent Advances in Core-Collapse Supernova Theory
Rochester Institute of Technology
résumé :
For approximately half a century, core-collapse supernovae have posed a vexing puzzle for theorists despite being a major ingredient (and uncertainty) in fields ranging from stellar and galaxy evolution to the interstellar medium. Historically, advances in core-collapse theory have been linked to advances in computing power and software. Supernovae are inherently multi-dimensional objects in which neutrino transport, gravity, hydrodynamic instabilities and convection play important roles. Three-dimensional simulations incorporating sufficient physical fidelity require extensive high-performance computing resources and codes efficient enough to use the associated architecture. In this talk, I will highlight recent advances in the field. In particular, I will discuss the dependence of spatial dimension on the viability of the delayed-neutrino mechanism and how pulsar kicks naturally arise from core collapse.
Vendredi 5 octobre 2012, 14h00
Salle de l'atelier, Paris
The debris disk -- terrestrial planet connection
Observatoire de Bordeaux
résumé :
The eccentric orbits of the known extrasolar giant planets provide evidence that most planet-forming environments undergo violent dynamical instabilities. We numerically simulate the impact of giant planet instabilities on planetary systems as a whole. We find that populations of inner rocky and outer icy bodies are both shaped by the giant planet dynamics and are naturally correlated. The orbital distributions of outer planetesimal disks can be drastically altered by the giant planets; in the most extreme cases they can be converted into 100 AU-scale isotropic clouds (aka "mini Oort clouds"). Strong instabilities - with very eccentric surviving giant planets - completely clear out their inner and outer regions. In contrast, systems with stable or low-mass giant planets form terrestrial planets in their inner regions and outer icy bodies produce dust that is observable as debris disks at mid-infrared wavelengths. Fifteen to twenty percent of old stars are observed to have bright debris disks (at lambda ~ 70 microns) and we predict that these signpost dynamically calm environments that should contain terrestrial planets.
Vendredi 28 septembre 2012, 14h00
Salle de l'atelier, Paris
Numerical simulations of binary galaxy mergers: on the formation of early-type galaxies
Maxime BOIS
Observatoire de Paris/LERMA
résumé :
I will present a study on the formation of early-type galaxies (ETGs) through mergers with a sample of 70 high-resolution numerical simulations of binary mergers of disc galaxies and 16 simulations of ETG remergers. These simulations, designed to accompany observations and models conducted within the Atlas3D project, encompass various mass ratios, initial conditions and orbital parameters. I will show that binary mergers can produce both fast rotating and slow rotating galaxies. Most Slow Rotators formed in these binary disc mergers hold a stellar Kinematically Distinct Core (KDC) in their ~ 1-3 central kilo-parsec: these KDCs are built from the stellar components of the progenitors. Using the GalMer database of galaxy mergers, we can confirm the importance of the spin of the progenitors in the formation of the fast and slow rotating galaxies. With the implementation of stellar populations models, these simulations are also useful to constrain the formation and the growth of ETGs via dry galaxy mergers.

Vendredi 14 septembre 2012, 14h00
Salle de l'atelier, Paris
AGN feeding and feedback in Perseus A and other low-redshift active galaxies.
Mount Stromlo Observatory
résumé :
In this talk, I will mainly focus on our recent results for the near-infrared
molecular hydrogen emission in the central 3 x 3 arcsec of Perseus A (NGC 1275), which are based on data obtained with the Near-Infrared Integral Field Spectrograph (NIFS)
and the ALTAIR adaptive-optics system (Gemini North).
The data support a scenario in which shock-excited molecular hydrogen enters
into the circum-nuclear region of Perseus A in the form of one or more streamers
and settles into a turbulent, clumpy or unstable accretion disc in the inner
R ~ 50 pc. Our results include an estimate of the mass enclosed by the molecular
hydrogen disc, which will be discussed in view of the M-sigma relation and black
hole mass estimates from the literature. I will examine the relative importance
of jet feedback and accretion in driving shocks and turbulence in the
molecular-gas component of Perseus A. I will also review other modes of AGN
feedback, including examples from our ongoing optical IFU study of bright
type-1 Seyfert galaxies at z<0.06.
Vendredi 7 septembre 2012, 14h00
Salle de l'atelier, Paris
Hypervelocity Stars
Warren BROWN
résumé :
A massive black hole sits in the heart of the Milky Way. One
consequence of the black hole is that it ejects "hypervelocity stars" from
the Milky Way at ~1000 km/s velocities. We discovered the first
hypervelocity star in 2005, and since then our targeted survey has
discovered 20 unbound stars and a comparable number of possibly bound
hypervelocity stars. Recent results include new constraints on their
origin in the Milky Way, a surprising anisotropic spatial distribution
over the sky, and Hubble Space Telescope proper motion measurements that
may allow us constrain the shape and orientation of the Galactic
Vendredi 20 juillet 2012, 14h00
Salle de l'atelier, Paris
Tracking the Evolution of Strong, 1.5 < z < 4.5 CIV Absorbers with Thousands of Systems
résumé :
Spectroscopic surveys of quasars yield a random sample of
intervening absorbing gas clouds that can be used to constrain the
on-going and summative enrichment processes in the universe. The CIV
doublet has proven to be an important tracer of the IGM and its
evolution from z = 6 to 0. This transition has been well-studied at
high redshift because: it is a strong transition of a common metal; it
is observable outside the Ly-alpha forest, where it becomes easier to
identify; it redshifts into optical passbands for 1.5 < z < 4.5; and
it is a resonant doublet, which gives it distinctive characteristics
that enable surveys to be largely automated. We have vastly improved
the 1.5 < z < 4.5 absorber measurements by identifying over 15,000 CIV
systems from a survey of thousands of SDSS DR7 QSOs. No longer
dominated by Poisson errors, the CIV redshift (non)evolution stands
out clearly. For example, the shape of the CIV distribution has not
evolved over the 3 Gyr span of the sample, but there has been a
two-fold increase in the CIV number density (detected at > 30 sigma).
Since the strong CIV absorbers detectable in SDSS spectra likely arise
in the extended gaseous halos of galaxies, we can show that the change
in the number density is probably largely driven by the change in the
number of galaxies. We also constructed a uniform 0 < z < 6 dataset by
combining the SDSS survey with the z < 1 HST results (Cooksey et al.
2010) and the new z > 5 FIRE results (Simcoe et al. 2011). Thus, we
can compare apples-to-apples: the absorber density over time and the
CIV mass density evolution. This is the first in a series on our
surveys for various metal-line absorption systems in SDSS DR7 QSOs,
and we share some other unexpected discoveries.
Vendredi 6 juillet 2012, 14h00
Salle de l'atelier, Paris
University of Victoria, BC
Vendredi 6 juillet 2012, 14h00
Salle de l'atelier, Paris
Isotropic Heating of Cool-Core Galaxy Cluster via Reccurrent, Rapidly Reorienting AGN Jets
University of Victoria
résumé :
AGN jets are more than capable of staving of catastrophic cooling of the intracluster medium (ICM) in the cores of cool-core clusters. However, preventing catastrophic cooling requires the ICM to be heated nearly isotropically. Narrow bipolar jets are extremely inefficient at heating the gas in the transverse direction. Recent detailed studies of individual cool-core clusters show that successive generations of jet-lobe-bubbles are offset, often significantly, in the angular direction on the sky. Since the spin and the jet axises are one and the same, we interpret this as evidence that the spin axis of the supermassive black hole (SMBH) at the centers of these clusters is prone to change directions on timescales shorter than the gas cooling time in the cores. We argue that due to the extant conditions in the cluster cores, the SMBHs experience short stochastic episodes of enhanced accretion via thin, typically misaligned, accretion disks that, in turn, cause the black holeu2019s spin axis to slew and change direction rapidly. Our model not only explains how AGN byproducts can end up tracing a nearly isotropic angular distribution about the cluster center but also explains how AGN jets effect isotropic heating. Since SMBHs that host thin accretions will manifest as quasars, our model predicts that to the extent that the characteristics of the cluster population (i.e. the fraction of strong cool-core clusters, etc.) are comparable to those in the local universe, we expect a quasar at the centres of 1-2 systems within z < 0.5.
Vendredi 29 juin 2012, 14h00
Salle de l'atelier, Paris
ALMA pour chacun
Observatoire de Paris/LERMA
résumé :
After an overview of ALMA capabilities for the upcoming Phase-I deadline
of July 12, I will present a brief discussion of tools for the treatment of the
data available at the LERMA.
Vendredi 15 juin 2012, 14h00
Salle de l'atelier, Paris
Interactions non-thermiques entre étoiles massives et nuages moléculaires: du domaine du TeV au domaine submm.
résumé :
Les étoiles massives réagissent de multiples façons sur le milieu dans
lesquelles elles se sont formées, et notamment les nuages moléculaires.
Je me concentrerai aujourd'hui sur deux effets nouveaux: (i) au cours de
leur vie, leurs vents stellaires créent de vastes bulles de plasma chaud
visibles en rayons X diffus; à son tour, ce plasma réagit avec les
nuages moléculaires froids environnants, donnant naissance à des
réactions d'échange de charge à leur contact, vues sous forme de raies X
non-thermiques particulières; (ii) à la fin de leur vie, leur explosion
en supernova créent des ondes de choc, qui accélèrent des particules.
Celles-ci interagissent avec les nuages moléculaires, pour donner à
haute énergie des photons gamma (GeV-TeV), détectés par HESS et Fermi,
mais aussi à basse énergie, provoquant localement une forte
surionisation des nuages moléculaires, récemment détectée dans le
domaine submillimétrique, via les radicaux DCO+ et HCO+.
Vendredi 4 mai 2012, 14h00
Salle de l'atelier, Paris
Eliminating the impossible: confronting theory and observations of high-redshift galaxies
résumé :
Galaxy evolution is fortunate to be a data-rich research field. But the constant stream of new results can bring with it some difficulties. When data are rare, there is at least time to become familiar with the competing theories before each new result is published. Conversely, when theories are regularly adapted in response to each new set of observations, it can be difficult to extract from literature the physical explanations that have, or have not, been ruled out. With this in mind, we review key issues arising from the latest high-redshift galaxy surveys, but focus on reexamining the observational samples in terms of the simple, physical limits from which our complex galaxy formation models are built:
- Could they even have formed according to standard cosmology, and natural cooling and infall thresholds?
- How would they continue to evolve under certain limiting cases of mass accretion?
- And what fraction of the familiar, low-redshift population might these
first glimpses of the early Universe really represent?
Vendredi 6 avril 2012, 14h00
Salle de l'atelier, Paris
The far-infrared view of AGNs: Nuclear activity, secular star formation and (a lack of) mergers
James Mullaney
résumé :

The tight links between supermassive black hole (SMBH) and galaxy spheroid mass observed in the local Universe strongly suggests they have grown concurrently. However, the large-scale mechanisms that drive SMBH and galaxy growth remain poorly understood, especially at high redshifts. While major-mergers between galaxies provide a natural route to intense star-formation and rapid SMBH growth, observations of the high redshift Universe have begun to question the importance of such dramatic events, instead suggesting that most stellar mass was put in place via slower, "secular" processes. In this talk, I will explore SMBH growth in terms of this secular vs. major-merger view of galaxy evolution in an attempt to determine how today's SMBHs acquired their mass.

Vendredi 16 mars 2012, 14h00
Salle de l'atelier, Paris
Une expérience de vie dans l’espace: Des graines de plantes exposées pendant 18 mois à l’extérieur de la Station Spatiale Internationale
Sydney LEACH
Observatoire Paris-Meudon/LERMA
résumé :
La vie, est-elle arrivée sur Terre de l’espace ? Notre expérience spatiale représente un test (partial) de cette hypothèse [1]. Dans le cadre du projet EXPOSE de l’ESA nous avons exposé des graines d’Arabidopsis Thaliana (l’Arabette des Dames) et de Nicotiana tabacum (tabac)
à l’extérieure de l’ISS pendant 18 mois. Nous avons mesuré la capacité de ces graines de germer au retour sur Terre, graines ayant subis pendant 18 mois les effets du rayonnement solaire, rayons cosmiques, variations de température et vide spatial existant à 440 km d’altitude. Les effets de ces divers paramètres ont été évalués. Les résultats ont montré qu’une graine de plante, ou une entité biologique similaire, pourrait survivre un transfert directe de Mars à la Terre ayant lieu en une année, durée courte, mais statistiquement possible.

Ce travail, qui comporte également des expériences de simulation au laboratoire, a été effectué, en collaboration intense, par David Tepfer (D.R. INRA) et Andreja Zalar (Docteur de l’ Univ.Versailles), biologistes des plantes à l’INRA de Versailles, et avec l’aide de Norbert Champion (LERMA, Obs.Meudon) et Soeren Hoffmann (ASTRID Synchrotron, Aarhus, Danemark).

[1] D.Tepfer, S.Leach, « Plant Seeds as Model Vectors for the Transfer of Life Through Space », Astrophys.Space Sci. 306 (2006) 69.
Vendredi 9 mars 2012, 14h00
Salle de l'atelier, Paris
Tackling the Galactic plane with Planck, Herschel and Gaia
CESR, Toulouse
résumé :
In most domains of astrophysics, knowledge of the distances to the objects
being studied is crucial. This is the case for stars in our Galaxy, which
will be spectacularly addressed by the upcoming Gaia mission. Currently,
two satellites are observing the Milky Way's interstellar medium (ISM) :
Herschel and Planck. The views that we are getting of the dust and gas are
at higher resolution and sensitivity to anything previously available.
However, in the plane of our Galaxy, the emission comes from a large range
of distances and environments and is concentrated in a relatively thin strip.
Not only do we need distances to infer physical sizes, masses and other
parameters of sources being studied, but we need to separate the different
contributions to the observed signal if we hope to perform a robust analysis.
I will present work that I have undertaken to explore the third dimension in
Galaxy, using stellar observations as well as thermal dust emission and radio
emission from the ISM. I will also discuss the long awaited ESA mission Gaia,
and its implications for the study of the ISM.
Vendredi 2 mars 2012, 14h00
Salle de l'atelier, Paris
Évolution physico-chimique des galaxies dans la seconde partie de l'histoire de l'univers
IRAP, Toulouse
résumé :
Les grands relevés spectro-photométriques comme le VVDS permettent
de déterminer la distance de milliers de galaxies et d'apporter des
contraintes sur les modèles cosmologiques et l'évolution des propriétés
globales des galaxies (luminosité, masse, environnement, ...).
Mais les observations et notamment les spectres de ces galaxies
contiennent beaucoup plus d'informations permettant d'affiner plus
précisément les modèles d'évolution physique et chimique.
Je ferai le point sur quelques résultats récents et sur les perspectives
attendues avec les nouveaux instruments (MUSE, EMIR, Euclid, ...).
Vendredi 17 février 2012, 14h00
Salle de l'atelier, Paris
Stellar Tidal Streams in nearby galaxies
Max-Planck-Institut fur Astronomie, Heidelberg
résumé :
Within the hierarchical framework for galaxy formation, minor merging
and tidal interactions are expected to shape large galaxies to this
day. As part of a pilot survey, we have carried out ultra-deep,
wide-field imaging of some isolated spiral galaxies in the local
universe with data taken at small (0.1 to 0.5-meter diameter),
robotic telescopes that provide exquisite surface brightness
sensitivity. Our observational effort has led to the discovery of
previously undetected giant stellar structures in the halos of
these spiral galaxies, likely associated with debris from tidally
disrupted satellites. Our collection of galaxies presents an
assortment of tidal phenomena exhibiting strikingly diverse
morphological characteristics. Our preliminary comparison with
available stellar halo simulations set in a Lambda-Cold Dark Matter
cosmology suggests that this extraordinary variety of morphological
specimens detected in our survey could represent one of the first
comprehensive pieces of evidence to support that the hierarchical
formation scenarios predicted by these theoretical models apply
generally to galaxies similar to the Milky Way in the Local Volume.
Finally, I will also present the discovery of a tidal stream around
a nearby dwarf irregular galaxy with our small telescopes. Followup
observations with Subaru telescope show this stream completely
resolved into stars, providing observational evidence of a minuscule
merger in a LMC-type system in the local universe. This result
suggests that dwarf accretion could play an important role in the
star formation history and evolution of nearby dwarf galaxies.
Vendredi 10 février 2012, 14h00
Salle de l'atelier, Paris
Dark matter in galaxies: results and prospects
Gianfranco GENTILE
Université de Gent, Belgique
résumé :
The dark matter particle has not (yet?) been detected, and
its nature is still unknown. Lambda Cold Dark Matter (LambdaCDM) is
the currently favoured paradigm for structure formation in the
Universe: it is very successful in explaining the observations of
large scale structures in the Universe, but on galaxy scales there is
a discrepancy between observations and (dark matter only) LambdaCDM
predictions. I will introduce the dark matter problem in galaxies, I
will talk about advances that have been made in deriving the
distribution of dark matter in galaxies, and I will discuss future
projects that will enable us to shed more light on dark matter,
including the HALOGAS (Westerbork Hydrogen Accretion in LOcal
GAlaxieS) survey, the deepest HI survey of a sizeable sample of spiral
Lundi 6 février 2012, 14h00
Salle de l'atelier, Paris
!!! Jour inhabituel !!!
Outbursts from Supermassive Black Holes seen in the Hot Atmospheres of Early-type Galaxies, Galaxy Groups, and RIch Clusters
William FORMAN
résumé :
We discuss the effects of supermassive black hole (SMBH) outbursts on
the hot atmospheres of early type galaxies, galaxy groups, and galaxy
clusters whose atmospheres are only seen through X-ray
observations. The discussion is motivated by the need to understand
the separation of galaxies into their two fundamental classes - the red
and "dead" early type galaxies with little star formation and the
blue clound (spiral, actively star forming galaxies). We show the
detailed effects of the outbursts from the supermassive black hole in
M87 at the center of the Virgo cluster using Chandra and XMM-Newton
observations including buoyant bubbles of relativistic plasma produced
by the central SMBH, uplifted filaments of X-ray emitting gas, and the
Mach 1.2 shock. We present the results from a large survey of more
than 100 early type galaxies observed with Chandra. From the X-ray
images, we identify the sample of hot coronae that show gas cavities,
estimate cavity ages, and compute the mechanical power needed to
inflate the cavities. From the X-ray luminosities, we derive
Eddington ratios and briefly discuss the accretion mode for these
low-luminosity actice galactic nuclei. We compare the early-type
galaxy AGN to their more powerful counterparts in rich clusters.
Finally, we show the dramatic effects that SMBH outbursts can have on
galaxy-scale hot coronae.
Vendredi 3 février 2012, 14h00
Salle de l'atelier, Paris
Molecular gas structure on GMCs scales in the nearby galaxy M51
Gaëlle DUMAS
Iram, Grenoble
résumé :
The Plateau de Bure Arcsec Whirlpool Survey (PAWS, PI: E. Schinnerer) has imaged
the CO(1-0) emission in the central 8 kpc of the nearby spiral galaxy M51.
Our final data is a combination of the IRAM 30m single-dish and Plateau de Bure
interferometer observations, reaching a linear resolution about 45pc and with
sensitivity to giant molecular clouds (GMCs) above 10^5 Msun. Thanks to such
high quality data, we are able to study the structure of the molecular gas from
galactic scales (e.g. spiral arms) down to a single GMC. In particular we study
the correlations between the CO(1-0), IR and Radio Continuum emissions on
different spatial scales and investigate the physical processes behind these
observed correlations. In this talk, I will present the first results of such
an analysis and their consequences on star formation estimation within
the galactic disk. I will then review the other prospects of this ambitious
project, discussing in particular how high resolution mm observations (e.g.
with ALMA) will further the study of the interstellar medium
in nearby galaxies and its role in star formation.
Vendredi 13 janvier 2012, 14h00
Salle de l'atelier, Paris
Black Hole Fueling and AGN Feedback: Toward a deeper physical understanding
Gregory NOVAK
Observatoire de Paris/LERMA
résumé :
We have performed axisymmetric hydrodynamic simulations black hole
fueling and feedback in a massive galaxy. The effects of the central
black hole on the temperature and momentum of galactic gas resulting
from both radiative and mechanical feedback (in the form of a
broad-line wind) are treated carefully using a detailed and physically
well-motivated prescription. The simulations cover a range of length
scales from ~1 pc to ~100 kpc. We carefully treat the forces on the
gas due to dust opacity in the UV, optical, and IR bands from photons
generated by both stars and the central AGN. We include a
prescription for angular momentum transport, allowing us to consider
galaxies with large specific angular momenta (disk galaxies) in the
axisymmetric code. We consider the case of including steady
cosmological infall of cold gas, as well as the case of the rapid
removal of the angular momentum of a cold gas disk to mimic the effect
of a galaxy merger. We find that the black hole accretion rate
depends strongly on the inner radius of the simulation, implying that
physical processes that operate on infalling gas between 1 and 100 pc
have an important effect on the true black hole accretion rate and the
resulting feedback processes.
Vendredi 6 janvier 2012, 14h00
Salle de l'atelier, Paris
Escaping atmospheres of exoplanets: towards hot neptunes, super-earths, and Earth-like planets
IPAG -- Institut de planétologie et d'astrophysique de Grenoble
résumé :
Nearly 700 extrasolar planets have been detected so far and an intense
characterisation effort has been undertaken to unveil the atmospheric
properties of some of these distant worlds seen in transit accross
their stars. A large number of transiting exoplanets are found in extreme irradiation environments, very close to their stars, and the question arise
of whether the atmospheres of these planets remain stable or get blown away.
In particular, it is surmised that the recently detected rocky super-earths
(1-10 Earth masses) can be evaporation remnants of once more massive planets
completely eroded by the extreme stellar irradiation. Detecting the extended
atmospheres of exoplanets in different mass regimes and measuring their
mass loss rates and efficiencies are key steps towards the understanding of
the atmospheric dynamics and properties of low-mass exoplanets. I will review
the results we have obtained with HST on the atmospheric evaporation of
transiting exoplanets, on both observational and theoretical sides. Steping
back into the habitable zones, I will finally discuss the prospects about
atmospheric characterisation for Earth-size planets (such as Kepler-22b) and
how the upcoming transit of Venus in June 2012 could be a Rosetta stone to
interpret the future transmission spectra of Earth-like exoplanets that could
be obtained in the next decade."
Vendredi 16 décembre 2011, 14h00
Salle de l'atelier, Paris
Study of prestellar dense core collapse and fragmentation using radiation-magneto-hydrodynamics
résumé :
It is established that stars form within molecular clouds via
gravitational collapse. I will present radiation-magneto-hydrodynamics
calculations of low-mass and massive dense core collapse, focusing on
the first collapse and the first hydrostatic core (first Larson core)
formation. The influence of magnetic field and initial mass on the
fragmentation properties will be investigated. In the first part,
I will briefly present the numerical method I use. In the second part
reporting low-mass dense core collapse calculations results,
synthetic observations of spectral energy distributions will be
derived, as well as classical observational quantities such as the
bolometric temperature and luminosity. I will show how the dust
continuum can help to target first hydrostatic cores and to state
about the nature of VeLLOs. Last, I will present synthetic
ALMA observation predictions of first hydrostatic cores which may
give an answer, if not definitive, to the fragmentation issue at
the early Class 0 stage.

In the third part, I will report the recent results of
radiation-magneto-hydrodynamics calculations in the context of
high mass star formation, using for the first time a self-consistent
model for photon emission (i.e. via thermal emission and in
radiative shocks) and with the high resolution necessary to resolve
properly magnetic braking effects and radiative shocks on scales
<100 AU (Commercon, Hennebelle & Henning ApJL 2011). In this study,
we investigate the combined effects of magnetic field, turbulence,
and radiative transfer on the early phases of the collapse and the
fragmentation of massive dense cores (M=100 Msun). We identify
a new mechanism that inhibits initial fragmentation of massive
dense cores, where magnetic field and radiative transfer interplay.
We show that this interplay becomes stronger as the magnetic field
strength increases. We speculate that highly magnetized massive
dense cores are good candidates for isolated massive star formation,
while moderately magnetized massive dense cores are more
appropriate to form OB associations or small star clusters.
Vendredi 9 décembre 2011, 14h00
Salle de l'atelier, Paris
Multiwavelength properties of distant galaxies under the cosmic microscope
CRAL, Lyon I
résumé :
I will present recent observational results obtained on the physics of
distant galaxies (~ 1<z<6) taking benefit of massive clusters used as
natural instruments. Gravitational magnification enables us to resolve
even the most distant objects, and to probe intrinsically fainter
sources at high redshift. This allows us to do the kind of research
that will only be achievable with the ELTs, but using
ground-based 8-10 m class telescopes and their current or
soon-forthcoming instruments. More precisely, after introducing
the topic and the use of massive clusters for their strong lensing
effect, I will present our ongoing work on strongly lensed LBGs in
the optical and near-infrared. In a second part, I will focus
on recent results obtained with Herschel, PdBI and SMA at
far-infrared / sub-mm and millimeter wavelengths, which enlarge
our panorama of the distant galaxy population.
Vendredi 25 novembre 2011, 14h00
Salle de l'atelier, Paris
A smooth and fast end of the dark ages of the universe
Felix Mirabel
résumé :
After a spectacular birth, with the glow of the Big Bang fading away and the first atoms of hydrogen being formed, our Universe quickly became a dull place, with the first stars and galaxies yet to appear. How long these cosmic dark ages lasted and how they came to an end are questions of outmost importance in cosmology.
Based on recent theoretical and observational results, I will show that besides the ultraviolet radiation from primordial massive stars, X-rays and relativistic jets from their black hole fossils, must have played an important role at the dawn of the Universe. In this context, contrary to the prevailing view that the early Universe had a "Swiss-cheese"-like appearance that lasted several hundred million years, the dark ages may have had a smooth and fast end. These could be good news for the future probes of the dark ages by means of the redshifted 21 cm line of atomic hydrogen.
Vendredi 18 novembre 2011, 14h00
Salle de l'atelier, Paris
What are the star-forming gas properties in local galaxies?
Estelle BAYET
résumé :
We currently have only a restricted picture of the star formation activity in
galaxies, partly because we don't have sufficient resolution to locate
properly sites of star formation beyond the Local Group. Interferometers such
as IRAM-PdBI, CARMA, SMA, and especillay ALMA now, are changing this picture.

In complement, I will show that we can actually improve our understanding of
the processes of star-formation in galaxies with single-dish telescopes, using
strategically key chemical tracers. More precisely, I will present work on the
determination of the ISM properties in local starburst, spirals, AGN-dominated
galaxies and early-type galaxies. Then, I will present some ISM properties in
'exotic' or more disturbed places such as in cosmic-ray dominated galaxies
and in cooling flows seen in the Perseus cluster.

I will also show some recent results acquired using interferometers enhancing
our knowledge to the finest scale possible today.
Mercredi 16 novembre 2011, 14h00
Salle des séminaire de l'IAP, Paris
Jour et lieu inhabituels
Galaxy Formation at its Most Active Phase
Avishai Dekel
The Hebrew University of Jerusalem
résumé :
Using hydro-cosmological simulations and analytic modeling, we attempt solid
predictions for the formation of massive galaxies at high redshift within the
LCDM cosmology. The emerging picture highlights the formation of galaxies
at the nodes of the cosmic web. These galaxies are steadily fed by cold
streams along dark-matter filaments, which are observable in Lyman-alpha.
The streams, including a smooth component and merging galaxies, penetrate
through hot gas in dark-matter halos to form extended, turbulent, rotating
disks and bulges with central black holes. The streams transport angular
momentum from large distances into the disk that grows inside out, but a significant exchange of angular momentum occurs in the greater disk vicinity.
The intense gas input drives a self-regulated, violent gravitational disk
instability with transient features and giant clumps, where stars form rapidly.
The violent instability induces quick migration to the center, complementing
mergers as a mechanism for spheroid formation and for feeding AGN. We address
the implications of this developing picture on different aspects of galaxy
formation theory and its observable features.
Vendredi 28 octobre 2011, 14h00
Salle de l'atelier, Paris
La formation d'étoiles dans les galaxies chimiquement jeunes du Groupe Local
Pierre Gratier
résumé :
La variété de galaxies dans le Groupe Local rend possible l'étude du milieu interstellaire et de la formation d'étoiles dans des conditions différentes de celles trouvées dans la Voie Lactée, tout en conservant une grande résolution spatiale grâce à leur proximité. Nous avons étudié le milieu interstellaire d galaxies du Groupe Local, M33 et NGC6822, dont les métallicités sont inférieur d'un facteur 2 à 3 à celle du soleil et qui sont respectivement dix fois et ce fois moins lumineuses que la Voie Lactée. Nos observations de la transition J=2- monoxyde de carbone, avec une résolution suffisante pour résoudre les nuages moléculaires géants, fournissent la première carte du milieu moléculaire de NG et la cartographie de M33 avec la meilleure combinaison de résolution et de sensibilité. Nous présentons également une cartographie haute résolution du mi atomique de M33 à partir d'une mosaïque intérférométrique dans la raie à 21cm l'ensemble du disque de la galaxie. Combinées avec des données allant de l'ultraviolet à l'infrarouge lointain, ces observations permettent l'étude du interstellaire et de la formation d'étoiles à des échelles allant du nuage individuel à la galaxie dans son ensemble. Ces deux objets, chimiquement jeun semblent convertir l'hydrogène moléculaire en étoiles plus rapidement que les grandes galaxies spirales comme la Voie Lactée. Est-ce à rapprocher du taux é de formation d'étoiles dans les galaxies de l'univers plus jeune (z~0.5-1), également riches en gaz et bleues comme M33 et NGC6822 ? Un soin particulier apporté pour tenter de mesurer la masse de dihydrogène, difficile dans ce type d'objet, à l'échelle de la galaxie ainsi qu'à l'échelle du nuage. Une méthode d'identification automatique et de mesure des propriétés physiques des nuages moléculaires géants a permis d'obtenir, dans le cas de M33, le plus grand cata de nuage moléculaires dans une galaxie extérieure. Il en résulte que les nuage M33 et de NGC 6822 ont, en moyenne, une largeur de raie plus faible, pour une donnée, que les nuages de la Voie Lactée. Dans M33, la fraction de petits nua augmente significativement avec le rayon galactocentrique. Au moins un sixièm nuages moléculaires géants ne sont pas associés à de la formation stellaire (détectée) mais nous n'avons pas identifié de caractéristiques physiques particulières pour ces nuages.
Vendredi 21 octobre 2011, 14h00
Salle de l'atelier, Paris
High-resolution Spectroscopic Studies of Diffuse Molecular Clouds
Steve Federman
Univ. Texas
résumé :
High spectral resolution allows us to discern the number of clouds along the line of
sight, their kinematics, the relative importance of turbulent motions, and chemical
signatures, including isotope ratios. Three recently completed projects will be
described. Absorption from atomic and molecular gas toward stars in the Pleiades
provided details of the interaction between the cluster and interstellar material.
It appears that the UV radiation from the stars has heated the gas sufficiently that
CH+ was formed in observable quantitites. In another study based on UV measuerments
with HST on CO and with FUSE on H2, the trends between CO abundance and H2 were
examined, as was the fractionation between 12C16O and 13C16O. The third study
involved 12 and 13 isotopoloques of CH+ and CN and the rotational temperatures of
12C14N and 13C14N. Here, we found that the 12CH+/13CH+ ratio appears to be
essentially constant within about a kpc of the Sun and that CN excitation
predomiantly arises from the Cosmic Background.
Jeudi 22 septembre 2011, 14h00
Salle de l'entresol à l'IAP, Paris
Attention ! Jour inhabituel
News from ATLAS3D: a new view at the nearby galaxy population
ESO, Garching
résumé :
I will briefly describe the goals and strategy of the Atlas3D project
which targeted a complete volume-limited sample of 260 nearby early-type
galaxies. I will summarise the main (mostly published) results, and then
update the audience with the most recent advances obtained within the
context of that project: this leads us to a significantly revised view
of the nearby population of galaxies.
Vendredi 1 juillet 2011, 14h00
Salle de l'atelier, Paris
On the Meaning of General Covariance and the Relevance of Observers in General Relativity
Luis Herrera
Université de Caracas
résumé :
Since the appearance of General Relativity, its intrinsic general covariance has been very often misinterpreted as implying that physically meaningful quantities (and conclusions extracted from the theory) have to be absolutely independent on observers. This incorrect point of view is sometimes expressed by discarding the very concept of observer in the structure and applications of the theory. As we shall stress in this seminar, through some examples, the concept of observer is as essential to General Relativity as it is to any physical theory.
Jeudi 30 juin 2011, 14h00
Salle de l'atelier, Paris
Attention ! Jour inhabituel
Formation de H2O dans les conditions du milieu interstellaire
Gianfranco Vidali
Physics Department, Syracuse University, Etats-Unis
résumé :
We studied the formation of water on dust grain analogs using isotopically labeled atomic hydrogen and oxygen beams. We detect the formation of water and intermediate products. As dust analogs, we used amorphous silicate films; for comparison, experiments done on a single crystal silicate were also carried out. The samples were characterized by in-situ infrared spectroscopy and ex-site atom force microscopy. Implications of these results on the formation of water in ISM environments will be discussed.
Vendredi 24 juin 2011, 14h00
Salle de l'atelier, Paris
Reporté à la rentrée !
Turbulence in the Molecular Interstellar Medium
Pierre Hily-Blant
résumé :
The Interstellar Medium is both the medium between the stars and the place of birth of the stars. At the Galactic scale, the ISM is best understood as a turbulent place. On smaller scales, the cold ISM, where stars form, is likely also turbulent. Words of caution arise from the lack of a definitive proof for the turbulent nature of the ISM which itself amounts to the lack of a complete theory of turbulence. Not only is the ISM turbulent: magnetic fields are now estimated strong enough to imprint their configuration on the cold matter distribution. If a theory of turbulence is still lacking, a theory of compressible, magnetized turbulence may seem unachievable. Even though, understanding turbulence in the cold ISM is required for a proper understanding of the cycle of matter in the ISM. The last decade has fostered direct numerical simulations of the physics of the ISM. Comparisons between theoretical predictions, numerical results, and observational constraints is becoming a tool to study the complex interplay of physics and chemistry in the ISM. I will review the general arguments that support a turbulent view of the ISM, with special emphasis towards the cold molecular ISM. Some consequences of turbulence in the ISM will be presented in the view of recent Herschel satellite results, whereas the Planck mission will provide constraints to the interplay between turbulence and magnetic fields. The soon-opening (extremely) high angular resolution era will certainly bring new constraints on fundamental questions regarding the nature of the interstellar turbulence and the structure of the ISM.

Vendredi 10 juin 2011, 14h00
Salle de l'atelier, Paris
Constraining the very high-redshift tail of Submillimeter Galaxies
H. Dannerbauer
CEA, Saclay
résumé :
The last years impressively showed that to understand the assembly and formation of massive galaxies, it is critical to study Submillimeter Galaxies (SMGs). Since their first detection more than ten years ago, several hundred dust-enshrouded high-z sources have been selected through ground-based submm/mm imaging with bolometer cameras like SCUBA, LABOCA, AzTEC or MAMBO, opening a new exciting era in observational cosmology and giving an important route to investigate star formation and the formation of spheroids in the distant universe. One of the current key topics in galaxy formation and assembly is to obtain an accurate estimate of the number of SMGs at z>4, thus to get a complet census of the star-formation at the most distant redshifts and add missing bits on the obscured universe at extreme redshifts to the Lilly-Madau Plot. I will discuss different approaches to find dusty starbursts at redshifts beyond z=4 and present our on-going Herschel and IRAM efforts on this research topic. Finally, I will discuss the prospects of ALMA studies on dusty starbursts in the very high-z universe.
Mercredi 1 juin 2011, 14h00
Salle de l'atelier, Paris
Attention ! Jour inhabituel
High Energy Density Plasmas: X-ray Sources and Plasma Jets
David Hammer
Cornell University
résumé :
The Cornell University Laboratory of Plasma Studies makes use of 1 trillion watt, 1 million ampere pulsed power generators for fundamental studies and applications of high energy density plasmas in several configurations. Some are as high density and temperature as $10^{22}/cm^3$ and 1.5 keV, respectively, with very short life, $<<$ 1 ns. Others last as long as a few hundred nanoseconds at somewhat lower density and temperature and take the form of plasma jets that can interact with other plasmas. These have been used for studies of the interaction of plasma jets with a cross wind in a configuration with dimensionless parameters that are of interest to the astrophysics community. The very short-lived, hot plasma is an extremely bright x-ray source that has been used for x-ray absorption spectroscopy in high energy density plasmas and could be used to determine the radiation transfer properties of materials that make up such astrophysical objects as supernova remnants.
Vendredi 20 mai 2011, 14h00
Salle de l'atelier, Paris
Probing the Epoch of Reionization by identifying (lensed) high-redshift galaxies
Jean-Paul Kneib
Laboratoire d'Astrophysique Marseille
résumé :
I will review the recent work conducted in the last year regarding the detection and study of the distant high-redshift (z>6) galaxies. Of particular importance is the recent installation of the WFC3 camera onboard of Hubble which InfraRed imaging channel is delivering the deepest images ever obtained in the 1-1.6 micron window. The new HST images are allowing to identify a large number of high redshift galaxy candidates that can then be follow-up in spectroscopy with 8-10m telescopes. I will discuss whether these findings can account from the reionization of the Universe, and what are the current observational limitations that can be improved with current and future facilities.
Please see http://www.spacetelescope.org/news/heic1106/
Vendredi 6 mai 2011, 09h30
Salle Danjon, Paris
Horaire et lieu inhabituels
Journée des doctorants

Séminaires à venir

Vendredi 23 avril 2021, 14h00
Visioconférence, VIDEO
A stellar graveyard in the core of a globular cluster
résumé :
The ubiquity of supermassive black holes in massive galaxies suggests the existence of intermediate-mass ones (IMBHs) in smaller systems. However, IMBHs are at best rare in dwarf galaxies and not convincingly seen in globular clusters. We embarked on a search for such an IMBH in a very nearby core-collapsed globular cluster, NGC 7397. For this we ran extensive mass-orbit modeling with our Bayesian MAMPOSSt-PM code that fits mass and velocity anisotropy models to the distribution of observed tracers in 4D projected phase space. We used a combination of proper motions from HST and Gaia, supplemented with redshifts from MUSE. We found very strong Bayesian evidence for an excess of unseen mass in the core of the cluster amounting to 1 to 2% of the cluster mass. But surprisingly, we found rather strong evidence that this excess mass is not point-like but has a size of roughly 3% of that of the cluster. Our conclusion is robust to our adopted surface density profile and on our modeling of the velocity anisotropy, as the data suggest isotropic orbits throughout the cluster. It is also robust to our use of one or two classes of Main Sequence stars (given the mass segregation in collisional systems such as clusters), as well as on our filtering for quality data. The expected mass segregation suggests that the excess mass is made of objects heavier than Main Sequence stars: white dwarfs, neutron stars and possibly stellar black holes, all of which lost their orbital energy by dynamical friction to end up in the cluster core. I will discuss the evidence for and against the possibility that most of the unseen mass in the center is in the form of such black holes, as well as the consequences of this intriguing possibility.
Tous les séminaires...