The fraction of early-type galaxies in low redshift groups and clusters
  of galaxies
  We examine the fraction of early-type (and spiral) galaxies found in groups
and clusters of galaxies as a function of dark matter halo mass. We use
morphological classifications from the Galaxy Zoo project matched to halo
masses from both the C4 cluster catalogue and the Yang et al (2007) group
catalogue. We find that the fraction of early-type (or spiral) galaxies remains
constant (changing by less than 10%) over three orders of magnitude in halo
mass (13<log MH/Msol/h<15.8). This result is insensitive to our choice of halo
mass measure, from velocity dispersions or summed optical luminosity.
Furthermore, we consider the morphology-halo mass relations in bins of galaxy
stellar mass M*, and find that while the trend of constant fraction remains
unchanged, the early-type fraction amongst the most massive galaxies (11<log
M*/Msol/h <12) is a factor of three greater than lower mass galaxies
(10<logM*/Msol/h<10.7). We compare our observational results with those of
simulations presented in De Lucia et al (2011), as well as previous
observational analyses, and semi-analytic bulge (or disc) dominated galaxies
from the Millennium Simulation. We find the simulations recover similar trends
as observed, but may over-predict the abundances of the most massive bulge
dominated (early-type) galaxies. Our results suggest that most morphological
transformation is happening on the group scale before groups merge into massive
clusters. However, we show that within each halo a morphology-density relation
remains: it is summing the total fraction to a self-similar scaled radius which
results in a flat morphology-halo mass relationship.
