  Green valley galaxies represent the population that is likely to transition
from the star-forming to the quiescent phases. To investigate the role of the
environment in quenching star formation, we use the wide-field data from the
Hyper Suprime-Cam Strategic Subaru Proposal survey to quantify the frequency of
green valley galaxies in different environments and their redshift evolution.
We find that the green valley fraction, in general, is less than 20% in any
redshift and environment. The green valley fraction, when normalized to the
total population, is higher in the field than that in groups or clusters and
decreases with a decreasing redshift and increasing mass. The lower fraction of
transitional galaxies in denser environments could be a consequence of the lack
of star-forming galaxies, which could be the progenitors of green valley
galaxies. To assess the effect of the environment on star formation quenching,
we define the effective green valley fraction as the ratio of the number of
green valley galaxies to that of nonquiescent galaxies only. The effective
green valley fraction for field galaxies is lower than that for group or
cluster galaxies, which reveals a strong positive mass dependence and mild
redshift evolution. Moreover, the specific star formation rate (sSFR) is
reduced by 0.1-0.3 dex in groups or clusters. Our results thus imply that an
ongoing slow quenching process has been acting in the dense environment since
z~1.
