zCOSMOS survey galaxy groups: exploring the effect of group environment on galaxy properties

The evolution of galaxies in groups may have important implications for the global evolution of the galaxy population as a whole. The fraction of galaxies bound in groups at z ∼ 0 is as high as 60% and many processes operating in groups may concur in shaping galaxy evolution. The rich zCOSMOS spectroscopic data (about 20000 galaxies with IAB ≤ 22.5 up to z ∼ 1) and its excellent group catalog (∼ 200 groups with more that 5 members up to z ∼ 1) coupled with the wide photometric coverage of the COSMOS survey, can shed new light on this topic, enabling us to study in a continuous way, up to z ∼ 1, the complex interplay between environment and galaxy evolution. In this thesis I present the new results I have obtained on this topic by exploring the group-centric dependence of galaxy colors, masses, morphologies and star formation. In brief, by building two composite groups at intermediate (0.2 ≤ z ≤ 0.45) and high (0.45 < z ≤ 0.8) redshifts, I was able to study in detail how galaxy stellar masses, colors, morphologies, and spectral features vary as a function of the distance from the group center. My analysis was performed in narrow bin of stellar masses/colors, in order to disentangle the obvious galaxy stellar mass/color dependencies. To build the composite group I developed an algorithm to incorporate the galaxies brighter than IAB = 22.5 and missing a secure spectroscopic redshift, thus improving the statistics of the sample. To confidently determine all galaxy projected distance and rescale them into the composite group, I defined a new centering technique. From the color/mass analysis I found that the evolution of most massive galaxies (log(Mgal/M⊙) > 10.6) is mainly driven by internal processes, as no strong group-centric environment dependence is visible. For galaxies of lower masses (9.8 ≤ log(Mgal/M⊙) ≤ 10.6) there is a radial depen- dence in the changing mix of red and blue galaxies, red galaxies residing preferentially in the group center. Such dependence is most evident in poor groups, whereas richer groups do not display any obvious color trend. Interestingly mass segregation shows the opposite behavior: it is visible only in rich groups, while poorer groups have a a constant mix of galaxy stellar masses as a function of radius. The morpho-spectral analysis showed the presence of a mild morphological segregation at fixed galaxy stellar mass, with massive early-type galaxies preferentially located in the core of groups. Galaxies with 9.8 ≤ log(Mgal/M⊙) ≤ 10.6 exhibit the strongest morphological differences between group and field environment. These galaxies also have an excess of red-passive spirals in the group with respect to the field. Despite the presence of a significant segregation of the spectral properties of group galaxies, such that the typical core galaxy have less intense emission lines, the star forming galaxies share the same level of activity at fixed stellar mass irrespective of the environment they reside in. This findings can be explained in a simple scenario where color/SFR and mass segregation originates from different physical processes. Mass segregation is driven by dynamical phenomena within groups, and therefore its presence/absence in rich/poor groups is a possible indication that poorer groups start to assemble later in cosmic time than richer structures. The parallel absence/presence of color segregation in rich/poor groups hints to the fact that nurture effects are still in action in poorer structures, whereas in richer systems are already largely over, so that all galaxies are red irrespective of their position within the group (at least down to the galaxy stellar masses we explored). Poorer groups hold the smoking gun of environmental effects in action superimposed to secular galaxy evolution: galaxies display gradually redder colors as a consequence of the still recent accretion history of these groups. The physical processes causing these environmental effects should act on rather short timescales, 1.5-2 Gyrs, because they are not able to erase the strinking bi-modality of galaxy color distribution and, moreover, we are not able to observed blue active galaxies showing less intense star formation activity in groups than in the field