The colour of galaxies in distant groups
  (Abridged) We present new optical and near-infrared imaging for a sample of
98 spectroscopically-selected galaxy groups at 0.25<z<0.55. We measure accurate
colours for group members and the surrounding field population, statistically
complete above a stellar mass limit of M=1E10 Msun. The overall colour
distribution is bimodal in both the field and group samples; but at fixed
luminosity the fraction of group galaxies populating the red peak is larger, by
20+/-7 per cent, than that of the field. In particular, group members with
early-type morphologies, as identified in HST imaging, exhibit a tight red
sequence, similar to that seen for more massive clusters. We show that
approximately 20-30 per cent of galaxies on the red sequence may be
dust-reddened galaxies with non-negligible star formation and early-spiral
morphologies. This is true of both the field and group sample, and shows little
dependence on near infrared luminosity. Thus, the fraction of bright group
members with no sign of star formation or AGN activity is 54+/-6 per cent. Our
field sample, which includes galaxies in all environments, contains 35+/-3 per
cent of such inactive galaxies, consistent with the amount expected if all such
galaxies are located in groups and clusters. This reinforces our earlier
conclusions, that dense environments at z<0.5 are associated with a premature
cessation of star formation in some galaxies; in particular we find no evidence
for significantly enhanced star formation in these environments. Simple galaxy
formation models predict a quenching of star formation in groups that is too
efficient, overpopulating the red sequence. Attempts to fix this by increasing
the timescale of this quenching equally for all group members distorts the
colour distribution in a way that is inconsistent with observations.
