Testing Cosmological Models against the Abundance of Damped Lyman‐Alpha Absorbers

We calculate the number of damped Lyα absorbers expected in various popular cosmological models as a function of redshift and compare our 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 simulations of a standard cold dark matter (CDM) model. Because of this calibration, our approach makes more realistic assumptions about the absorption properties of collapsed objects than previous, analytic calculations of the damped Lyα abundance. CDM models with Ω 0 = 1, H 0 = 50 km s -1 Mpc -1 , baryon density Ω b = 0.05, and scale-invariant primeval fluctuations reproduce the observed incidence and redshift evolution of damped Lyα absorption to within observational uncertainty, for both COBE normalization (σ 8 = 1.2) and a lower normalization (σ 8 = 0.7) that better matches the observed cluster abundance at z = 0. A tilted ( n = 0.8, σ 8 = 0.7) CDM model tends to underproduce absorption, especially at z = 4. With COBE normalization, a CDM model with Ω 0 = 0.4, Ω Λ = 0.6 gives an acceptable fit to the observed absorption; an open CDM model is marginally acceptable if Ω 0 ≥ 0.4 and is strongly inconsistent with the z = 4 data if Ω 0 = 0.3. Mixed dark matter models tend not to produce sufficient absorption, being roughly comparable to tilted CDM models if Ω ν = 0.2 and failing drastically if Ω ν = 0.3.