Cosmological constraints from high-redshift damped Ly alpha systems

Any viable cosmological model must produce enough structure at early epochs to explain the amount of gas associated with high-redshift damped Ly alpha systems. We study the evolution of damped Ly alpha systems at z greater than or equal to 2 in cold dark matter (CDM) and in cold plus hot dark matter (CDM+HDM) models using both N-body and hydrodynamic simulations, Our approach incorporates the effects of gas dynamics, and we find that all earlier estimates that assumed that all the baryons in dark matter halos would contribute to damped Ly alpha absorption have overestimated the column density distribution, f(N), and the fraction of neutral dense gas, Omega(g), in damped Ly alpha systems. The differences are driven by ionization of hydrogen in the outskirts of galactic halos and by gaseous dissipation near the halo centers, and they tend to exacerbate the problem of late galaxy formation in CDM+HDM models. We include systems only up to the highest observed column density N similar to 10(21.8) cm(-2) in the estimation of Omega(g) for a fair comparison with data. If the observed f(N) and Omega(g) inferred from a small number of confirmed and candidate absorbers are robust, the amount of gas in damped Ly alpha systems at high redshifts in the Omega(v) = 0.2 CDM+HDM model falls well below the observations.