Implications of the remarkable homogeneity of galaxy groups and clusters
  We measure the diversity of galaxy groups and clusters with mass M>1E13/h
Msun, in terms of the star formation history of their galaxy populations, for
the purpose of constraining the mass scale at which environmentally-important
processes play a role in galaxy evolution. We consider three different group
catalogues, selected in different ways, with photometry and spectroscopy from
the Sloan Digital Sky Survey. For each system we measure the fraction of
passively-evolving galaxies within R200 and brighter than either Mr=-18 (and
with z<0.05) or Mr=-20 (and z<0.1). We use the (u-g) and (r-i) galaxy colours
to distinguish between star-forming and passively-evolving galaxies. By
considering the binomial distribution expected from the observed number of
members in each cluster, we are able to either recover the intrinsic scatter in
this fraction, or put robust 95% confidence upper-limits on its value. The
intrinsic standard deviation in the fraction of passive galaxies is consistent
with a small value of <0.1 in most mass bins for all three samples. There is no
strong trend with mass; even groups with M=1E13/h Msun are consistent with such
a small, intrinsic distribution. We compare these results with theoretical
models of the accretion history to show that, if environment plays a role in
transforming galaxies, such effects must occur first at mass scales far below
that of rich clusters, at most M=1E13 Msun.
