We study the formation of damped Lyα and Lyman-limit absorbers in a hierarchical clustering scenario using a gasdynamieal simulation of an Ω = 1 cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped Lyα absorption, N_(H I) approx> 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) approx< N_(H I) approx< 10~(20.2) cm~(-2), develops on lines of sight that pass through the outer parts of such objects or near the centers of smaller protogalaxies. The number of Lyman-limit systems is less than observed, while the number of damped Lyα systems is quite close to the observed abundance. Damped absorbers are typically ~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.
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