THE EPOCH OF GALAXY FORMATION

We use a semianalytic model of galaxy formation in hierarchical cluste ring theories to interpret recent data on galaxy formation and evoluti on, focusing primarily on the recently discovered population of Lyman- break galaxies at z similar or equal to 3. For a variety of cold dark matter (CDM) cosmologies, we construct mock galaxy catalogs subject to selection criteria identical to those applied to the real data. We fi nd that the expected number of Lyman-break galaxies is very sensitive to the assumed stellar initial mass function and to the normalization of the primordial power spectrum. For reasonable choices of these and other model parameters, it is possible to reproduce the observed abund ance of Lyman-break galaxies in CDM models with Omega(0) = 1 and Omega (0) < 1. The characteristic masses, circular velocities, and star form ation rates of the model Lyman-break galaxies depend somewhat on the v alues of the cosmological parameters, but are broadly in agreement wit h available data. These galaxies generally form from rare peaks at hig h redshift, and as a result their spatial distribution is strongly bia sed, with a typical bias parameter of b similar or equal to 4 and a co moving correlation length of r(0) similar or equal to 4 h(-1) Mpc. The typical sizes of these galaxies, similar to 0.5 h-l kpc, are substant ially smaller than those of present-day bright galaxies. In combinatio n with data at lower redshifts, the Lyman-break galaxies can be used t o trace the cosmic star formation history. We compare theoretical pred ictions for this history with a compilation of recent data. The observ ational data match the theoretical predictions reasonably well, both f or the distribution of star formation rates at various redshifts and f or the integrated star formation rate as a function of redshift. Most galaxies tin our models and in the data) never experience star formati on rates in excess of a few solar masses per year. Our models predict that even at z = 5, the integrated star formation rate is similar to t hat measured locally, although less than 1% of all the stars have form ed prior to this redshift. The weak dependence of the predicted star f ormation histories on cosmological parameters allows us to propose a f airly general interpretation of the significance of the Lyman-break ga laxies as the first galaxy-sized objects that experience significant a mounts of star formation. These galaxies mark the onset of the epoch o f galaxy formation that continues into the present day. The basic ingr edients of a consistent picture of galaxy formation may well now be in place.