Direct observational evidence for a large...

Direct observational evidence for a large transient galaxy population in groups at 0.85

Abstract

(abridged) We introduce our survey of galaxy groups at 0.8515 members. The dynamical mass estimates are in good agreement with the masses estimated from the X-ray luminosity, with most of the groups having 131E10.1 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.