MODELING GALAXY FORMATION IN EVOLVING DARK MATTER HALOS

A simple semianalytic model is used to study the radiative cooling of gas of an evolving distribution of cold dark matter halos. Two related models are used to describe the evolving distribution of collisionless dark matter: the Press-Schechter formalism and a recently developed Monte Carlo model. The Monte Carlo method follows the merging of halos and hence extends the fully analytic treatment in several ways. First, plausible estimates of halo lifetimes are made. Second, the local depletion of the primordial gas is followed in each halo as progressively more gas cools and forms stars. Finally, limits are set on the merging of protogalaxies. It is shown that, in the absence of any form of energy input, an unacceptably large fraction of the baryons will cool and condense in low-mass halos at high redshift. In order to counteract efficient cooling at early times, it is postulated that energy input by supernovae associated with the first stars formed in these halos is sufficient to unbind the gas in halos with circular velocities less than a critical value V(crit). Estimates of the present-day luminosity function calculated using this model suggest V(crit) almost-equal-to 200 km s-1 and hence that self-regulating or self-limiting star formation operates even in bright L* galaxies.