X-ray Groups of Galaxies at 0.5<z<1 in zCOSMOS: Increased AGN Activities
  in High Redshift Groups
  We present a photometric and spectroscopic study of galaxies at 0.5<z<1 as a
function of environment based on data from the zCOSMOS survey. There is a fair
amount of evidence that galaxy properties depend on mass of groups and
clusters, in the sense that quiescent galaxies prefer more massive systems. We
base our analysis on a mass-selected environment using X-ray groups of galaxies
and define the group membership using a large number of spectroscopic redshifts
from zCOSMOS. We show that the fraction of red galaxies is higher in groups
than in the field at all redshifts probed in our study. Interestingly, the
fraction of [OII] emitters on the red sequence increases at higher redshifts in
groups, while the fraction does not strongly evolve in the field. This is due
to increased dusty star formation activities and/or increased activities of
active galactic nuclei (AGNs) in high redshift groups. We study these
possibilities using the 30-band photometry and X-ray data. We find that the
stellar population of the red [OII] emitters in groups is old and there is no
clear hint of dusty star formation activities in those galaxies. The observed
increase of red [OII] emitters in groups is likely due to increased AGN
activities. However, our overall statistics is poor and any firm conclusions
need to be drawn from a larger statistical sample of z~1 groups.
