Cosmological Constraints from High-Redshift Damped Lyman-Alpha Systems

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 redshifts z ≥ 2 in cold dark matter (CDM) and cold+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 which 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 only include systems up to the highest observed column density N ∼ 1021.8 cm 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. Submitted to The Astrophysical Journal Letters Subject headings: cosmology: theory — dark matter — galaxies: formation — hydrodynamics – quasars: general — absorption lines 1 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104; [email protected] Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 Presidential Faculty Fellow Department of Astronomy, University of California, Santa Cruz, CA 95064 Department of Astronomy, Ohio State University, Columbus, OH 43210 Department of Astronomy, University of Washington, Seattle, WA 98195