DISC GALAXIES AT Z=O AND AT HIGH-REDSHIFT - AN EXPLANATION OF THE OBSERVED EVOLUTION OF DAMPED LY-ALPHA ABSORPTION SYSTEMS

The analysis of disc formation in this paper is based on the White & R ees model, in which disc galaxies form by the continuous cooling and a ccretion of gas within a merging hierarchy of dark matter haloes. A si mple Kennicutt law of star formation for discs, based on a single-flui d gravitational stability model, is introduced. Since the gas supply i n the disc is regulated by infall from the surrounding halo, the gas i s always maintained at a critical threshold surface density Sigma(c), where Sigma(c) proportional to V-c/R. Chemical enrichment of the discs occurs when the surrounding hot halo gas is enriched with heavy eleme nts ejected during supernova explosions. This gas then cools on to the disc, producing a new generation of metal-rich stars. We show that mo dels of this type can reproduce many of the observed properties of a t ypical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ag es and metallicities of solar neighbourhood stars. In particular, we f ind that the rapid early enrichment predicted by our model solves the classic G-dwarf problem. In addition, we are able to account for some of the global trends in the properties of disc galaxies, such as the o bserved relation between galaxy luminosity, metallicity and gas conten t. We then use our models to make inferences about the properties of d isc galaxies at high redshift. Because the overall mass distribution i n the Universe shifts to smaller haloes at higher redshifts, and these smaller haloes contain less luminous, more gas-rich galaxies, we find that the total neutral hydrogen density Omega (H I) increases at high er z. The predicted increase is mild, but is roughly consistent with t he latest derivation of Omega (H I) as a function oft by Storrie-Lomba rdi & MacMahon. Our models are also able to account for some of the ot her trends seen in the high-redshift data, including the increase in t he number of high-column-density systems at high redshift, as well as the metallicity distribution of damped Ly alpha systems at z similar t o 2-3. Finally, one completely general prediction of these models is t hat, at high redshift, the galaxies that produce the damped Ly alpha a bsorption will typically be smaller, more compact and less luminous th an disc galaxies at the present epoch.