The nature of galaxy bias and clustering

We have used a combination of high resolution cosmological N-body simulatio ns and semianalytic modelling of galaxy formation to investigate the proces ses that determine the spatial distribution of galaxies in cold dark matter (CDM) models and its relation to the spatial distribution of dark matter. The galaxy distribution depends sensitively on the efficiency with which ga laxies form in haloes of different mass. In small mass haloes, galaxy forma tion is inhibited by the reheating of cooled gas by feedback processes, whe reas in large mass haloes, it is inhibited by the long cooling time of the gas. As a result, the mass-to-light ratio of haloes has a deep minimum at t he halo mass, similar to 10(12) M., associated with L-* galaxies, where gal axy formation is most efficient. This dependence of galaxy formation effici ency on halo mass leads to a scale-dependent bias in the distribution of ga laxies relative to the distribution of mass. On large scales, the bias in t he galaxy distribution is related in a simple way to the bias in the distri bution of massive haloes. On small scales, the correlation function is dete rmined by the interplay between various effects including the spatial exclu sion of dark matter haloes, the distribution function of the number of gala xies occupying a single dark matter halo and, to a lesser extent, dynamical friction. Remarkably, these processes,conspire to produce a correlation fu nction in a flat, Ohm(0) = 0.3, CDM model that is close to a power law over nearly four orders of magnitude in amplitude. This model agrees well with the correlation function of galaxies measured in the automated-plate measur ement survey. On small scales, the model galaxies are less strongly cluster ed than the dark matter, whereas on large scales they trace the occupied ha loes. Our clustering predictions are robust to changes in the parameters of the galaxy formation model, provided only those models which match the bri ght end of the galaxy luminosity function are considered.