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

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Abstract
We study the formation of damped Lyα and Lyman-limit absorbers in a hierarchical clustering scenario using a gasdynamical 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, NH I 1020.2 cm-2, arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman-limit absorption, 1017 cm-2 NH I 1020.2 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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