Disc galaxies at z = 0 and at high redshift: an explanation of the observed evolution of damped Lya absorption systems

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Abstract
The analysis of disc formation in this paper is based on the White & Rees model, in which disc galaxies form by the continuous cooling and accretion of gas within a merging hierarchy of dark matter haloes. A simple Kennicutt law of star formation for discs, based on a single-fluid gravitational stability model, is introduced. Since the gas supply in the disc is regulated by infall from the surrounding halo, the gas is always maintained at a critical threshold surface density Σc, where Σc∞Vc/R. Chemical enrichment of the discs occurs when the surrounding hot halo gas is enriched with heavy elements ejected during supernova explosions. This gas then cools on to the disc, producing a new generation of metal-rich stars. We show that models of this type can reproduce many of the observed properties of a typical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ages and metallfcities of solar neighbourhood stars. In particular, we find 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 observed relation between galaxy luminosity, metallicity and gas content. We then use our models to make inferences about the properties of disc galaxies at high redshift. Because the overall mass distribution in 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 Ω(Hi) increases at higher z. The predicted increase is mild, but is roughly consistent with the latest derivation of Ω (H i) as a function of z by Storrie-Lombardi & MacMahon. Our models are also able to account for some of the other trends seen in the high-redshift data, including the increase in the number of high-column-density systems at high redshift, as well as the metallicity distribution of damped Lyα systems at z∼2–3. Finally, one completely general prediction of these models is that, at high redshift, the galaxies that produce the damped Lyα absorption will typically be smaller, more compact and less luminous than disc galaxies at the present epoch.
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