Are galaxy properties only determined by the dark matter halo mass?
  Conditional luminosity function (CLF) of dark matter halos supersedes simple models on the number of galaxies as a function of the halo mass, or the so-called halo occupation number, by assigning a luminosity distribution to those galaxies. Suggestions have now been made that both the clustering strength and properties of hosted galaxies of a given dark matter halo depends also on the age and the environment of that halo in addition to halo mass. Based on simple CLF models, we find that at least one suggestion that made use of central galaxy properties may be affected by uncertainties in the group catalog from which clustering properties were measured to address the age dependence. Establishing how galaxy properties of fixed mass halos change with the environment or the age is challenging, if not impossible, given uncertainties in group and cluster catalogs. It may be possible to address this issue through statistics based on a combined study of luminosity and color distributions, and luminosity- and color-dependent galaxy clustering, all of them as a function of the galaxy overdensity, though no single statistic is likely to provide the ultimate answer. The suggestion that the age dependence of the halo bias invalidates the analytical models of the galaxy distribution is premature given the improvements associated with modeling approaches based on conditional functions. 
Asantha Cooray