Scientific rationale

Understanding the formation and evolution of galaxy groups is crucial to solving the general problem of galaxy formation, as groups contain most of the galaxies in the universe at the present day, and most of the universal stellar mass is formed in groups. In addition, in the current bottom-up paradigm of structure formation, galaxy groups are the building blocks of more massive systems: they merge to eventually form clusters. As structures grow, galaxies join more and more massive systems, spending most of their life in galaxy groups before entering eventually the cluster environment. Groups of galaxies are special systems in an observational sense as well, as it is possible to probe both the stellar and hot gas components of these systems directly.


AGN feedback and massive galaxies

In recent years, a growing number of authors have argued, based on the results of semi-analytic models of galaxy formation as well as hydrodynamic simulations, that radio feedback from supermassive BHs plays a crucial role in regulating the star formation rates of massive galaxies and suppressing the onset of catastrophic cooling of the hot gas in groups and clusters. However, the observational results regarding galaxy groups are still inconclusive in this respect. Groups have low gas fractions within their inner region and the radio AGN outbursts should have significant impact on their X-ray properties. Therefore, groups that have experienced strong heating episodes would be X-ray faint and are difficult to be studied in detail in X-rays. These systems would also be underrepresented in any group samples selected in the X-ray or by X-ray cavities with the strong contrast. In addition, groups showing sign of cool core do not host usually strong radio AGN. Most of these radio outbursts should generate strong shocks in groups (Mach number > 2). However, all reported shocks in groups are weak ones with Mach number of 1.5 - 1.7.


Do group galaxies evolve faster?

In the last decade a substantial effort has been devoted to the study of high redshift groups to investigate the possibility of a differential evolution of group galaxies with respect to field galaxies. A significant step forward was achieved thanks to the advent of very deep multiwavelength surveys conducted on several blank fields. In this context, the main outcome of these surveys is that group galaxies show a much faster evolution with respect to the field galaxy. For instance, the formation of the galaxy red sequence, which leads to the local dichotomy between red and blue galaxies, happens earlier in groups than in the field especially at the high stellar masses. It seems also that group galaxies undergo a substantial morphological transformation. Indeed, groups at z~1 host a transient population of "red spirals" which is not observed in the field. In parallel, the star formation activity of group galaxies seems to be suppressed in a more efficient way since z~1 than in other environments.

The open questions
Where are we now? Are the X-ray observations deep enough to let us finally observe X-ray under-luminous groups with strong radio relics? Are there any group cool cores with strong radio AGN at the center? Can we find strong AGN shocks in groups? In other words, can we confirm the theoretical paradigm of massive galaxy formation and the crucial role of “radio mode” AGN feedback? Do the X-ray selection provide an unbiased view of the relation between AGN radio properties and X-ray group properties? Are the current observations deep enough to probe the faint X-ray groups or do we need to wait for eROSITA and Athena+?
Is the group environment able to influence the properties of the whole group galaxy population? If this is the case, what is the environmental process responsible for changing the group galaxy morphology and
quenching their star formation activity?


To answer these questions we organize a 4 days workshop with two aims:


- reviewing the galaxy group properties with a multiwavelength approach:

- from detailed X-ray observations to probe the properties of the hot intra-group gas

- to the optical- mid and far infrared wavelengths to probe the group galaxy population properties in terms of morphology and star formation activity

- down to the radio wavelengths to probe the incidence and the properties of the central radio AGN

- reviewing the current theoretical picture of group and galaxy formation and evolution:

- by looking at the details of the AGN feedback in comparison with observations

- by exploring the limits of the current models and find new solutions


The workshop will include the following sections:


- review of the theoretical predictions on central AGN

- hot gas properties connection (theory)

- limit and failure of the current model (theory)

- group scaling relations (observations)

- evolution of the group hot gas properties (theory and observations)

- high redshift group surveys (observations)

- deep X-ray group surveys (observations)

- deep radio group surveys (observations)

- AGN feedback in the group environment (theory and observations)

- Massive group central galaxies (theory and observations)

- Evolution of the galaxy population in groups (theory and observations)

- Star formation quenching in the group environment (theory and observations)

- Galaxy morphological transformation in groups (theory and observations)

Scientific Organizing Committee

P. Popesso (Excellence Cluster Universe, Garching)

V. Mainieri (European Southern Observatory, Garching)

A. Merloni (Max Planck fur extraterrestrische Physik, Garching)

K. Dolag (Ludwig-Maximilliam University, Munich)

A. Burkert (Ludwig-Maximilliam University, Munich)

J. Mohr (Ludwig-Maximilliam University, Munich)

D. Wilman (Max Planck fur extraterrestrische Physik, Garching)

M. Salvato (Max Planck fur extraterrestrische Physik, Garching)





Local Organizing Committee

P. Popesso (Excellence Cluster Universe, Garching)

G. Erfanianfar (Excellence Cluster Universe, Garching)

A. Concas (Excellence Cluster Universe, Garching)

L. Morselli (Excellence Cluster Universe, Garching)

A. Muller (Excellence Cluster Universe, Garching)

S. Lutz-Lampertseder (Excellence Cluster Universe, Garching)





Invited speakers

- Sun M.: review of group scaling relations
- Bower R: review of AGN feedback role in simulations
- Borgani S.: review of current status of the art simulations
- Best P.: review of radio AGN feedback
- Cen R.: review of quenching processes in groups
- Kovac K: review of environmental quenching
- Finoguenov A.: review of group X-ray survey
- Yang Y.: review of group optical surveys

- Silverman J.: review AGN and environment

- Leauthaud: review groups and AGN feedback

- Sanders J.: AGN feedback in clusters

- Robotham: review galaxy group optical selection

- Sean McGee: review evolution of the group galaxy population