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

Lyman limit and damped Lya absorption systems probe the distribution of collapsed, cold gas at high redshift. Numerical simulations that incorporate gravity and gasdynamics can predict the abundance of such absorbers in cosmological models. We develop a semianalytical method to correct the numerical predictions for the contribution of unresolved low-mass halos, and we apply this method to the Katz et al. simulation of the standard cold dark matter model () \ 1, h \ 0.5, and Using ) b \ 0.05, p 8 \ 0.7). this simulation and higher resolution simulations of individual low-mass systems, we determine the relation between a halos circular velocity and its cross section for producing Lyman limit or damped Lya v c absorption. We combine this relation with the Press-Schechter formula for the abundance of halos itself calibrated against the simulated halo populationin order to compute the number of absorbers per unit redshift. The resolution correction increases the predicted abundances by about a factor of 2 at z \ 2, 3, and 4, bringing the predicted number of damped Lya absorbers into quite good agreement with observations. Roughly half of these systems reside in halos with circular velocities km s~1, and v c 100 half in halos with 35 km km s~1. Halos with km s~1 typically harbor two or s~1 v c 100 v c [ 150 more systems capable of producing damped absorption. Even with the resolution correction, the predicted abundance of Lyman limit systems is a factor of 3 below observational estimates, signifying either a failure of the standard cold dark matter model or a failure of these simulations to resolve most of the systems responsible for Lyman limit absorption. By comparing simulations with and without star formation, we nd that the depletion of the gas supply by star formation a ects absorption-line statistics at z 2 for column densities exceeding cm~2 only, even though half of the cold, collapsed gas N H I \ 1022 has been converted to stars by z \ 2.