The epoch of galaxy formation

We use a semi-analytic model of galaxy formation in hierarchical clustering theories to interpret recent data on galaxy formation and evolution, focussing primarily on the recently discovered population of Lyman-break galaxies at z ≃ 3. For a variety of cold dark matter (CDM) cosmologies we construct mock galaxy catalogues subject to identical selection criteria to those applied to the real data. We find 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 abundance of Lyman-break galaxies in CDM models with Ω0 = 1 and with Ω0 < 1. The characteristic masses, circular velocities and star-formation rates of the model Lyman-break galaxies depend somewhat on the values of the cosmological parameters but are broadly in agreement with available data. These galaxies generally form from rare peaks at high redshift and, as a result, their spatial distribution is strongly biased, with a typical bias parameter, b ≃ 4, and a comoving correlation length, r0 ≃ 4h Mpc. The typical sizes of these galaxies, ∼ 0.5hkpc, are substantially smaller than those of present day bright galaxies. In combination with data at lower redshifts, the Lyman-break galaxies can be used to trace the cosmic star formation history. We compare theoretical predictions for this history with a compilation of recent data. The observational data match the theoretical predictions reasonably well, both for the distribution of star formation rates at various redshifts and for the integrated star formation rate as a function of redshift. Most galaxies (in our models and in the data) never experience star formation 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 that measured locally. The tail of star formation at high redshift produces enough radiation to, in principle, photoionize the intergalactic medium by z ≃ 5. The weak dependence of the predicted star formation rates on cosmological parameters allows us to propose a fairly general interpretation of the significance of the Lymanbreak galaxies as the first galaxy-sized objects that experience significant amounts of star formation. These galaxies mark the onset of the epoch of galaxy formation that continues into the present day. The basic ingredients of a consistent picture of galaxy formation appear to be in place.