Direct observational evidence for a large transient galaxy population in
  groups at 0.85<z<1
  (abridged) We introduce our survey of galaxy groups at 0.85<z<1, as an
extension of the Group Environment and Evolution Collaboration (GEEC). Here we
present the first results, based on Gemini GMOS-S nod-and-shuffle spectroscopy
of seven galaxy groups selected from spectroscopically confirmed, extended XMM
detections in COSMOS. In total we have over 100 confirmed group members, and
four of the groups have >15 members. The dynamical mass estimates are in good
agreement with the masses estimated from the X-ray luminosity, with most of the
groups having 13<log(Mdyn/Msun)<14. Our spectroscopic sample is statistically
complete for all galaxies with Mstar>1E10.1 Msun, and for blue galaxies we
sample masses as low as Mstar=1E8.8 Msun. Like lower-redshift groups, these
systems are dominated by red galaxies, at all stellar masses Mstar>1E10.1 Msun.
Few group galaxies inhabit the "blue cloud" that dominates the surrounding
field; instead, we find a large and possibly distinct population of galaxies
with intermediate colours. The "green valley" that exists at low redshift is
instead well-populated in these groups, containing ~30 per cent of galaxies.
These do not appear to be exceptionally dusty galaxies, and about half show
prominent Balmer-absorption lines. Furthermore, their HST morphologies appear
to be intermediate between those of red-sequence and blue-cloud galaxies of the
same stellar mass. We postulate that these are a transient population,
migrating from the blue cloud to the red sequence, with a star formation rate
that declines with an exponential timescale 0.6 Gyr< tau < 2 Gyr. Their
prominence among the group galaxy population, and the marked lack of blue,
star-forming galaxies, provides evidence that the group environment either
directly reduces star formation in member galaxies, or at least prevents its
rejuvenation during the normal cycle of galaxy evolution.
