Damped Lyman-Alpha and Lyman-Limit Absorbers in the Cold Dark Matter Model

We study the formation of damped \lya and Lyman limit absorbers in ahierarchical clustering scenario using a gas dynamical simulation of an $\Omega= 1$, cold dark matter universe. In the simulation, these high column densitysystems are associated with forming galaxies. Damped \lya absorption, $N_{HI}\simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near thecenters of relatively massive, dense protogalaxies. Lyman limit absorption,$10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, develops on lines ofsight that pass through the outer parts of such objects or near the centers ofsmaller protogalaxies. The number of Lyman limit systems is less than observed,while the number of damped \lya systems is quite close to the observedabundance. Damped absorbers are typically $\sim 10$ kpc in radius, but thepopulation has a large total cross section because the systems are much morenumerous than present day $L_*$ galaxies. Our results demonstrate that highcolumn density systems like those observed arise naturally in a hierarchicaltheory of galaxy formation and that it is now possible to study these absorbersdirectly from numerical simulations.Comment: compressed postscript, 12 pages including 2 embedded figures. A version that also includes embedded Figure 1, a 6 Mbyte color postscript image (which prints reasonable grey scale on a b/w printer) is available from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprints Submitted to ApJ Letter