THE PAN-STARRS1 MEDIUM-DEEP SURVEY: THE ROLE OF GALAXY GROU P ENVIRONMENT IN THE STAR FORMATION RATE VERSUS STELLAR MASS RELATION AND QUIESCENT F

Using a large optically-selected sample of field and group ga laxies drawn from the Pan-STARRS1 MediumDeep Survey (PS1/MDS), we present a detailed analysis of the specific star formation rate (SSFR) ‐ stellar mass (M∗) relation, as well as the quiescent fraction versus M∗ relation in different environments. While both the SSFR and the quiescent fraction depend strongly on stellar mass, the environment also plays an important role. Using this large galaxy sample, we confirm that the frac tion of quiescent galaxies is strongly dependent on environment at a fixed stellar mass, but that the amplitude an d the slope of the star-forming sequence is similar between the field and groups: in other words, the SSFR-densit y relation at a fixed stellar mass is primarily driven by the change in the star-forming and quiescent fractions between different environments rather than a global suppression in the star formation rate for the star- forming population. However, when we restrict our sample to the cluster-scale environments (M > 10 14 M⊙), we find a global reduction in the SSFR of the star forming sequence of 17% at 4σ confidence as opposed to its field counterpart. After removin g the stellar mass dependence of the quiescent fraction seen in field galax ies, the excess in the quiescent fraction due to the environment quenching in groups and clusters is found to increase with stellar mass, although deeper and larger data from the full PS1/MDS will be required to draw firm conclu sions. We argue that these results are in favor of galaxy mergers to be the primary environment quenching mechanism operating in galaxy groups whereas strangulation is able to reproduce the observed trend in the environment quenching efficiency and stellar mass relation seen in clusters. Our results also suggest that the relative importance between mass quenching and environment quenching depends on stellar mass ‐ the mass quenching plays a dominant role in producing quiescent galaxies for more massive galaxies, while less massive galaxies are quenched mostly through the environmental effect, with the transition mass around 1 - 2× 10 10 M⊙ in the group/cluster environment. Subject headings: galaxies:clustering - galaxies:evolution - galaxies: high-redshift - large-scale structure of Universe