Groups of galaxies: a key environment for galaxy evolution

Galaxy groups are the most common structure in the universe and, as they span a wide range in local density, showing properties which range from cluster-like to field-like represent the ideal place to investigate galaxy evolution. In particular, on these scales can be active fundamental pre-processes responsible of galaxy transformations; thus, they can have a profound effect on the overall galaxy population since about 50% of all galaxies are in groups by z ~ 0. The aim of the thesis has been to explore the properties of galaxies in groups and to understand whether and how they vary as function of global environment in which they reside. In this thesis I first present the construction and the characteristics of a new catalog of groups and galaxies representative of the general field population at low redshift, the Padova Millennium Galaxy and Group Catalogue (PM2GC). The group building method used to identify galaxy groups is based on a Friends-of-Friends algorithm which takes advantage of two available quantities in observational galaxy catalogs: the projected separation in the sky and the velocity difference in the redshift space. Non-group galaxies were subdivided into “binary” systems of two bright close companions, and “single” galaxies with no companion, in order to identify different environments useful scientific analysis. I compared galaxy groups properties with those of more isolated galaxies in other environments in PM2GC and with cluster galaxies from the WIde-field Nearby Galaxy cluster Survey (WINGS). I performed a morphological analysis studying the relation between morphology and mass among single, binaries, group and cluster galaxies. I found a very smooth trend of all morphological types with environment except for S0 galaxies in clusters. Moreover, I shown that the morphological fraction depends strongly on galaxy stellar mass at very high masses. Finally, I investigated the galaxy stellar mass function (MF) to understand if the observed variations in the galaxy morphology can be explained by the dependence of the galaxy mass itself on environment. I found that at low redshift, contrary to expectations, the mass function of general field is indistinguishable from that of galaxy clusters while the difference becomes evident only considering lowest mass haloes, i.e. single galaxies. I also found indication that there is a dependence of the upper mass limit of samples on environment with the most massive galaxies only hosted by the most massive haloes. All these results represent a very important indications that not only stellar mass but also the environment in which a galaxy resides play a role in galaxy evolution. Moreover, they highlight that exist a cluster specific effect which acts on the classes of spirals and S0s galaxies since a significant number of S0s in clusters has a different origin with respect to S0s in other environments. This means that S0 population in clusters probably are closely related to spiral one