Galaxy occupation statistics of dark matter haloes: observational results

We study the occupation statistics of galaxies in dark matter haloes using galaxy groups identified from the Two-degree Field Galaxy Redshift Survey with the halo-based group finder of Yang et al.. The occupation distribution is considered separately for early- and late-type galaxies, as well as in terms of central and satellite galaxies. The mean luminosity of the central galaxies scales with halo mass approximately as L-c proportional to M-2/3 for haloes with masses M < 10(13) h(-1) M-circle dot, and as L-c proportional to M-1/4 for more massive haloes. The characteristic mass of 10(13) h(-1) M-circle dot is consistent with the mass scale where galaxy formation models suggest a transition from efficient to inefficient cooling. Another characteristic halo mass scale, M similar to 10(11) h(-1) M-circle dot, which cannot be probed directly by our groups, is inferred from the conditional luminosity function (CLF) that matches the observed galaxy luminosity function and clustering. For a halo of given mass, the distribution of L-c is rather narrow. A detailed comparison with mock galaxy redshift surveys indicates that this implies a fairly deterministic relation between L-c and the halo mass. The satellite galaxies, however, are found to follow a Poissonian number distribution, in excellent agreement with the occupation statistics of dark matter subhaloes. This provides strong support for the standard lore that satellite galaxies reside in subhaloes. The central galaxies in low-mass haloes are mostly late-type galaxies, while those in massive haloes are almost all early types. We also measure the CLF of galaxies in haloes of given mass. Over the mass range that can be reliably probed with the present data, 13.3 less than or similar to log [M/( h(-1) M-circle dot)] less than or similar to 14.7, the CLF is reasonably well fitted by a Schechter function. Contrary to recent claims based on semi-analytical models of galaxy formation, the presence of central galaxies does not show up as a strong peak at the bright end of the CLF. The CLFs obtained from the observational data are in good agreement with the CLF model obtained by matching the observed luminosity function and large-scale clustering properties of galaxies in the standard Lambda cold dark matter model.