LERMA UMR8112

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.

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.