Damped Lyman-alpha systems and galaxy formation

We examine the constraints on theories of galaxy formation that are obtained from observations of damped Lyα systems, assuming they are gaseous disks in dark matter halos. Using the Press-Schechter formalism, we find that the mixed dark matter model, with OMEGAHDM_ = 0. 3, OMEGACDM_ = 0. 65, OMEGAb_ = 0. 05, h = 0. 5, and the COBE normalization, predicts too small a number of galactic halos at z ~ 3 to account for the total gaseous mass observed in damped systems, even under the assumption that all the gas in collapsed halos has settled into disks of neutral gas. The standard CDM model can account for the damped systems if the bias is b <~ 2; the same is true for the CDM model with a cosmological constant, if b <~ 1. 5 for LAMBDA = 0. 8. However, one still needs to assume that a substantial fraction of the baryons in collapsed halos at z~3 is in the form of neutral gas in disks. We also discuss the implications of the number of damped systems observed, for the radial distribution of the gas in disks.