SLOW QUENCHING OF STAR FORMATION IN OMEGAWINGS CLUSTERS: GALAXIES IN TRANSITION IN THE LOCAL UNIVERSE

The star formation quenching depends on environment, but a full understanding of what mechanisms drive it is still missing. Exploiting a sample of galaxies with masses M-* > 10(9.8) M-circle dot, drawn from the WIde-field Nearby Galaxy-cluster Survey (WINGS) and its recent extension OMEGAWINGS, we investigate the star formation rate (SFR) as a function of stellar mass (M-*) in galaxy clusters at 0.04 < z < 0.07. We use non-member galaxies at 0.02 < z < 0.09 as a field control sample. Overall, we find agreement between the SFR-M-* relation in the two environments, but detect a population of cluster galaxies with reduced SFRs, which is rare in the field. These transition galaxies are mainly found within the cluster virial radius (R-200), but they impact on the SFR-M-* relation only within 0.6R(200). The ratio of transition to pure star-forming galaxies strongly depends on environment, being larger than 0.6 within 0.3R(200) and rapidly decreasing with distance, while it is almost flat with M-*. As galaxies move downward from the SFR-M-* main sequence, they become redder and present older luminosity-and mass-weighted ages. These trends, together with the analysis of the star formation histories, suggest that transition galaxies have had a reduced SFR for the past 2-5 Gyr. Our results are consistent with the hypothesis that the interaction of galaxies with the intracluster medium via strangulation causes a gradual shut down of star formation, giving birth to an evolved population of galaxies in transition from being star forming to becoming passive.