The radial distribution of galaxies in groups and clusters
  We present a new catalogue of 55,121 groups and clusters centred on Luminous
Red Galaxies from SDSS DR7 in the redshift range 0.15<z<0.4. We provide halo
mass estimates for each of these groups derived from a calibration between the
optical richness of bright galaxies (M_r<-20.5) within 1 Mpc, and X-ray-derived
mass for a small subset of 129 groups and clusters with X-ray measurements. We
derive the mean (stacked) surface number density profiles of galaxies as a
function of total halo mass in different mass bins. We find that derived
profiles can be well-described by a projected NFW profile with a concentration
parameter (<c>~2.6) that is approximately a factor of two lower than that of
the dark matter (as predicted by N-body cosmological simulations) and nearly
independent of halo mass. Interestingly, in spite of the difference in shape
between the galaxy and dark matter radial distributions, both exhibit a high
degree of self-similarity. A self-consistent comparison to several recent
semi-analytic models of galaxy formation indicates that: (1) beyond ~0.3 r_500
current models are able to reproduce both the shape and normalisation of the
satellite profiles; and (2) within ~0.3 r_500 the predicted profiles are
sensitive to the details of the satellite-BCG merger timescale calculation. The
former is a direct result of the models being tuned to match the global galaxy
luminosity function combined with the assumption that the satellite galaxies do
not suffer significant tidal stripping, even though their surrounding dark
matter haloes can be removed through this process. Combining our results with
measurements of the intracluster light should provide a way to inform
theoretical models on the efficacy of the tidal stripping and merging
processes.
