TESTING COSMOLOGICAL MODELS AGAINST THE ABUNDANCE OF DAMPED LYMAN-ALPHA ABSORBERS

We calculate the number of damped Ly alpha absorbers expected in vario us popular cosmological models as a function of redshift and compare o ur predictions with observed abundances. The Press-Schechter formalism is used to obtain the distribution of halos with circular velocity in different cosmologies, and we calibrate the relation between circular velocity and absorption cross section using detailed gasdynamical sim ulations of a standard cold dark matter (CDM) model. Because of this c alibration, our approach makes more realistic assumptions about the ab sorption properties of collapsed objects than previous, analytic calcu lations of the damped Ly alpha abundance. CDM models with Omega(0) = 1 , H-0 = 50 km s(-1) Mpc(-1), baryon density Omega(b) = 0.05, and scale -invariant primeval fluctuations reproduce the observed incidence and redshift evolution of damped Ly alpha absorption to within observation al uncertainty, for both COBE normalization (sigma(8) = 1.2) and a low er normalization (sigma(8) = 0.7) that better matches the observed clu ster abundance at z = 0. A tilted (n = 0.8, sigma(8) = 0.7) CDM model tends to underproduce absorption, especially at z = 4. With COBE norma lization, a CDM model with Omega(0) = 0.4, Omega(Lambda) = 0.6 gives a n acceptable fit to the observed absorption; an open CDM model is marg inally acceptable if Omega(0) greater than or equal to 0.4 and is stro ngly inconsistent with the z = 4 data if Omega(0) = 0.3. Mixed dark ma tter models tend not to produce sufficient absorption, being roughly c omparable to tilted CDM models if Omega(nu) = 0.2 and failing drastica lly if Omega(nu) = 0.3.