A team leaded by Michael Balogh of the University of Waterloo, Ontario Canada, has presented the results of a survey of galaxy groups at 0.85 < z < 1, as an extension of the Group Environment and Evolution Collaboration (GEEC). 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. Like lower-redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar > 1010.1M⊙. Few group galaxies inhabit the “blue cloud” that dominates the surrounding field. The authors found there exists 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 about 30 per cent of the galaxies. These do not appear to be exceptionally dusty galaxies.
Furthermore, their HST morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. Unlike red-sequence galaxies, most of the green galaxies have a disk component, but one that is smaller and less structured than disks found in the blue cloud. 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 0.6Gyr < z < 2Gyr. Such galaxies may not be exclusive to the group environment, as we find examples also amongst the non-members. However, 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.
This work has been accepted to publication by the Monthy Notices of the Royal Astronomical Society (MNRAS). (read the article at arXiv)