  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 a
merging hierarchy of dark matter halos. A simple Kennicutt law of star
formation for disks, based on a single- fluid gravitational instability model
is introduced. Since the gas supply in the disk is regulated by infall from the
surrounding halo, the gas is always maintained at a critical threshold surface
density. Chemical enrichment of the disks occurs when the surrounding hot halo
gas is enriched with heavy elements ejected during surpernova explosions. This
gas 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 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 metallicities of solar neighbourhood stars. I then use the models to make
inferences about the properties of disk galaxies at high redshift. The total
neutral hydrogen density Omega(HI) increases at higher z. The predicted
increase is mild, but is roughly consistent with the latest derivation of
Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models
are also able to account for some of the other trends 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
Lyman-alpha systems at redshifts 2-3.
