The group galaxy population through the cosmic time

One of the most fundamental correlations between the properties of
galaxies in the local Universe is the so-called morphology-density
relation (Dressler 1980). A plethora of studies utilizing
multi-wavelength tracers of activity have shown that late type star
forming galaxies favour low density regions in the local Universe
(e.g. G´omez et al. 2003). In particular, the cores of massive
galaxy clusters are galaxy graveyards full of massive spheroids
that are dominated by old stellar populations. A variety of
physical processes might be effective in suppressing star formation
and affecting the morphology of cluster and group galaxies. Broadly
speaking, these can be grouped in two big families: (i)
interactions with other cluster members and/or with the cluster
gravitational potential and (ii) interactions with the hot gas that
permeates massive galaxy systems. Galaxy groups are the most common
galaxy environment in our Universe, bridging the gap between the
low density field and the crowded galaxy clusters. Indeed, as many
as 50%-70% of galaxies reside in galaxy groups in the nearby
Universe (Huchra & Geller 1982; Eke et al. 2004), while only a
few percent are contained in the denser cluster cores. In addition,
in the current bottom-up paradigm of structure formation, galaxy
groups are the building blocks of more massive systems: they merge
to form clusters. As structures grow, galaxies join more and more
massive systems, spending most of their life in galaxy groups
before entering the cluster environment. Thus, it is plausible to
ask if group-related processes may drive the observed relations
between galaxy properties and their environment. To shed light on
this topic we have built the largest X-ray selected samples of
galaxy groups with secure spectroscopic identification on the major
blank field surveys. For this purpose, we combine deep X-ray
Chandra and XMM data of the four major blank fields (All-wavelength
Extended Groth Strip International Survey (AEGIS), the COSMOS
field, the Extended Chandra Deep Field South (ECDFS), and the
Chandra Deep Field North (CDFN) ). The group catalog in each field
is created by associating any X-ray extended emission to a galaxy
overdensity in the 3D space. This is feasible given the extremely
rich spectroscopic coverage of these fields. Our identification
method and the dynamical analysis used to identify the galaxy group
members and to estimate the group velocity dispersion is
extensively tested on the AEGIS field and with mock catalogs
extracted from the Millennium Simulation (Springel et al. 2005).
The effect of dynamical complexity, substructure, shape of X-ray
emission, different radial and redshift cuts have been explored on
the LX −sigma relation. We also discover a high redshift group at
z~1.54 in the AEGIS field. This detection illustrates that
mega-second Chandra exposures are required for detecting such
objects in the volume of deep fields. We provide an accurate
measure of the Star Formation Rate (SFR) of galaxies by using the
deepest available Herschel PACS and Spitzer MIPS data available for
the considered fields. We also provide a well-calibrated estimate
of the SFR derived by using the SED fitting technique for
undetected sources in mid- and far-infrared observations. Using
this unique sample, we conduct a comprehensive analysis of the
dependence of the total SFR , total stellar masses and halo
occupation distribution (HOD) of massive galaxies (M*>10^10
M_sun) on the halo mass of the groups with rigorous consideration
of uncertainties. We observe a clear evolution in the level of star
formation (SF) activity in galaxy groups. Indeed, the total star
formation activity in high redshift (0.5 10^10.4−10.6 M_sun). Above
this limit, the galaxy SFR has a very weak dependence on the
stellar mass. This flattening, to different extent, is present in
all environments. At low redshift, group galaxies tend to deviate
more from the mean MS towards the region of quiescence with respect
to isolated and filament-like galaxies. This environment dependent
location of low redshift group galaxies with respect to the mean MS
causes the increase of the dispersion of the distribution of
galaxies around the MS as a function of the stellar mass. At high
redshift we do not find significant evidence for a differential
location of galaxies with respect to the MS as a function of the
environment. Indeed, in this case we do not observe a significant
increase of the dispersion of the distribution of galaxies around
the MS as a function of the stellar mass. We do not find evidence
for a differential distribution in the morphological type of MS
galaxies in different environments. Instead, we observe a much
stronger dependence of the mean S´ersic index on the stellar mass.
These results suggest that star formation quenching in group
galaxies is not due to galaxy structural transformations. It also
suggests that while morphology of MS galaxies is more stellar mass
dependent, star formation quenching is mostly environment
dependent. We conclude that the membership to a massive halo is a
key ingredient in the galaxy evolution and that this acts in terms
of star formation quenching in group sized halos.