The Pan-STARRS1 Medium-Deep Survey: The role of galaxy group environment
  in the star formation rate versus stellar mass relation and quiescent
  fraction out to $z \sim 0.8$
  Using a large sample of field and group galaxies drawn from the Pan-STARRS1
Medium-Deep Survey, we present the specific star formation rate (SSFR) -
stellar mass (M*) relation, as well as the quiescent fraction versus M*
relation in different environments. We confirm that the fraction of quiescent
galaxies is strongly dependent on environment at a fixed stellar mass, but that
the amplitude and the slope of the star-forming sequence is similar between the
field and groups: in other words, the SSFR-density 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 ($M>10^{14}M_{solar}$),
we find a global reduction in the SSFR of the star forming sequence of $17\%$
at 4$\sigma$ confidence as opposed to its field counterpart. After removing the
stellar mass dependence of the quiescent fraction seen in field galaxies, the
excess in the quiescent fraction due to the environment quenching in groups and
clusters is found to increase with stellar mass. 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
$1-2\times10^{10}M_{solar}$ in the group/cluster environment. (abridged)
