Damped Lyman-alpha and Lyman-limit absorbers in the cold dark matter model

We study the formation of damped Ly alpha and Lyman-limit absorbers in a hierarchical clustering scenario using a gasdynamical simulation of an Omega = 1 cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped Ly alpha absorption, N-HI greater than or similar to 10(20.2) cm(-2), arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman-limit absorption, 10(17) cm(-2) less than or similar to N-HI less than or similar to 10(20.2) cm(-2), develops on lines of sight that pass through the outer parts of such objects or near the cm centers of smaller protogalaxies. The number of Lyman-limit systems is less than observed, while the number of damped Ly alpha systems is quite close to the observed abundance. Damped absorbers are typically similar to 10 kpc in radius, but the population has a large total cross section because the systems are much more numerous than present-day L(*) galaxies. Our results demonstrate that high column density systems like those observed arise naturally in a hierarchical theory of galaxy formation and that it is now possible to study these absorbers directly from numerical simulations.