Cosmological Constraints from High-Redshift Damped L[CLC]yα[/CLC] Systems

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Abstract
Any viable cosmological model must produce enough structure at early epochs to explain the amount of gas associated with high-redshift damped Lyα systems. We study the evolution of damped Lyα systems at z ≥ 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 Lyα absorption have overestimated the column density distribution, f(N), and the fraction of neutral dense gas, Ωg, in damped Lyα 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 ~ 1021.8 cm-2 in the estimation of Ωg for a fair comparison with data. If the observed f(N) and Ωg inferred from a small number of confirmed and candidate absorbers are robust, the amount of gas in damped Lyα systems at high redshifts in the Ων = 0.2 CDM+HDM model falls well below the observations.
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