LERMA UMR8112

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.

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.

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.