Galaxies on the road to quenching

This thesis explores the properties of galaxies that reside in high density regions in the local Universe. The driving motivation is to shed light on how galaxies stop forming stars and the role played by the environment in this process. A wide variety of environmentallyrelated quenching mechanisms have been proposed and the smoking gun proving that these are actually transforming galaxies from being star forming to passive is the existence of a population of galaxies in transition with intermediate features. In this Thesis I will mainly focus on the properties of two population of galaxies: those that are forming stars at a reduced level compared to other galaxies at similar mass, and those that have recently and abruptly interrupted their star formation. The data exploited in this Thesis come from surveys which focus on different environments in the local Universe: the WIde-field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. 2006) with its recent extension OmegaWINGS (Gullieuszik et al., 2015a, Moretti et al. submitted) and the Padova Millennium Galaxy and Group catalogue (PM2GC, Calvi et al. 2011). The Thesis starts with a characterization of the WINGS survey and then focuses on the new OmegaWINGS data, illustrating the most important aspects of the photometric and spectroscopic observations. I will describe in detail my personal contribution to the survey, from the data analysis to the redshift and velocity dispersion measurements, to the membership assignments. I will also present the complete galaxy catalog built from the combination of the two cluster surveys considered. While halo mass estimates are quite easily obtained for clusters, at the group regime it is less straightforward to derive robust measurements. I will then present a procedure I developed to compute halo masses from observable quantities for the PM2GC. My approach exploits mock galaxy catalogs extracted from the De Lucia & Blaizot (2007) semi-analytic model, run on the Millennium Simulation (Springel et al. 2005). Subsequently, I will discuss the methods adopted to compute the galaxy stellar properties, exploiting both spectroscopic and photometric information. Using the data from the WINGS+OmegaWINGS surveys, I will present an analysis of the ongoing Star Formation Rate (SFR) and the Specific Star Formation Rate (SSFR) - stellar mass (M∗) relations in field and cluster mass limited samples. I contrast trends in the different environments and, in clusters, at different clustercentric distances. The main result is that in clusters a population of galaxies with a reduced SFR is detected, named transition galaxies, which is much rarer in the field. The spatial location of these galaxies, together with the analysis of the star formation histories, colors and average ages, suggest that transition galaxies have had a reduced SFR for the past 2-5 Gyr. This is compatible with a strangulation scenario, even if other processes like ram pressure stripping can not be excluded. I characterize the properties of post starburst (PSB) galaxies, which are galaxies that abruptly interrupted their star formation sometimes during the past 2Gyr), the other a rapid truncation of the star formation activity (t∼0.1Gyr). Comparing the fraction of PSBs to the fraction of galaxies in transition, it appears that the short timescale star-formation quenching channel contributes two times more than the long timescale one to the growth of the passive population in clusters. The emerging picture is that ram pressure stripping is probably the most successful process affecting star formation in clusters, followed by strangulation. Other mechanisms might play a role, even though they most likely take place in lower density environments.