Linking Early and Late Type Galaxies to their Dark Matter Haloes
  (Abridged) Using data from the 2dFGRS we compute the conditional luminosity functions (CLFs) of early- and late-type galaxies. These functions give the average number of galaxies with luminosity L that reside in a halo of mass M, and are a powerful statistical tool to link the distribution of galaxies to that of dark matter haloes. We find that the average mass-to-light ratios of dark matter haloes have a minimum of about 100 Msun/Lsun around a halo mass of about 3x10^{11} Msun. In addition, haloes with M < 10^{10} Msun are virtually devoid of galaxies, while clusters have b_J band mass-to-light ratios in the range 500-1000 Msun/Lsun. Finally, the fact that early-type galaxies are more strongly clustered than late-type galaxies requires that the fraction of late-type galaxies is a strongly declining function of halo mass. We also compute two-point correlation functions as function of both luminosity and galaxy type, and find a weak (strong) luminosity dependence for the late (early) type galaxies. Finally we compare our CLFs with predictions from several semi-analytical models for galaxy formation. As long as these models accurately fit the 2dFGRS luminosity function the agreement with our predictions is remarkably good. This indicates that we have recovered a statistical description of how galaxies populate dark matter haloes which fits nicely within the standard framework for galaxy formation. 
Frank C. van den Bosch (MPA), Xiaohu Yang (USTC) and H.J. Mo (MPA)