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

The analysis of disk formation is based on the White & Rees (1978) picture,in which disk galaxies form by continuous cooling and accretion of gas within amerging hierarchy of dark matter halos. A simple Kennicutt law of starformation for disks, based on a single- fluid gravitational instability modelis introduced. Since the gas supply in the disk is regulated by infall from thesurrounding halo, the gas is always maintained at a critical threshold surfacedensity. Chemical enrichment of the disks occurs when the surrounding hot halogas is enriched with heavy elements ejected during surpernova explosions. Thisgas then cools onto the disk producing a new generation of metal-rich stars. I first show that models of this type can reproduce many of the observedproperties of a typical spiral galaxy like the Milky Way, including its gas andstellar surface density profiles and the observed relationship between the agesand metallicities of solar neighbourhood stars. I then use the models to makeinferences about the properties of disk galaxies at high redshift. The totalneutral hydrogen density Omega(HI) increases at higher z. The predictedincrease is mild, but is roughly consistent with the latest derivation ofOmega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The modelsare also able to account for some of the other trends other trends seen in thehigh-redshift data, including the increase in the number of high column-densitysystems at high redshift, as well as the metallicity distribution of dampedLyman-alpha systems at redshifts 2-3.Comment: 14 pages with 8 figs included, uuencoded postscript file, to appear in the Proceedings of the Workshop on Coldgas at High Redshift, Kluwe