The Population of Damped Lyα and Lyman Limit Systems in the Cold Dark Matter Model

Lyman limit and damped Lyman-alpha absorption systems probe the distributionof collapsed, cold gas at high redshift. Numerical simulations that incorporategravity and gas dynamics can predict the abundance of such absorbers incosmological models. We develop a semi-analytical method to correct thenumerical predictions for the contribution of unresolved low mass halos, and weapply this method to the Katz et al. (1996) simulation of the standard colddark matter model ($\Omega=1$, $h=0.5$, $\Omega_b=0.05$, $\sigma_8=0.7$). Usingthis simulation and higher resolution simulations of individual low masssystems, we determine the relation between a halo's circular velocity $v_c$ andits cross section for producing Lyman limit or damped absorption. We combinethis relation with the Press-Schechter formula for the abundance of halos tocompute the number of absorbers per unit redshift. The resolution correctionincreases the predicted abundances by about a factor of two at z=2, 3, and 4,bringing the predicted number of damped absorbers into quite good agreementwith observations. Roughly half of the systems reside in halos with circularvelocities $v_c>100\kms$ and half in halos with $35\kms150\kms$ typically harbor two or more systems capable of producingdamped absorption. Even with the resolution correction, the predicted abundanceof Lyman limit systems is a factor of three below observational estimates,signifying either a failure of standard CDM or a failure of these simulationsto resolve the systems responsible for most Lyman limit absorption. Bycomparing simulations with and without star formation, we find that depletionof the gas supply by star formation affects absorption line statistics at$z>=2$ only for column densities exceeding $N_{HI}=10^{22} cm^{-2}$.Comment: AASlatex, 17 pages w/ 3 embedded ps figures. Submitted to Ap