Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 a
hierarchical clustering scenario using a gas dynamical simulation of an $\Omega
= 1$, cold dark matter universe. In the simulation, these high column density
systems are associated with forming galaxies. Damped \lya absorption, $N_{HI}
\simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near the
centers of relatively massive, dense protogalaxies. Lyman limit absorption,
$10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, 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 \lya systems is quite close to the observed
abundance. Damped absorbers are typically $\sim 10$ 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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The categories of this paper are: cond-mat
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The categories of this paper are: hep-lat
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The categories of this paper are: math-ph
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The categories of this paper are: eess
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The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
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The categories of this paper are: nucl-th
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The categories of this paper are: atom-ph
13
The categories of this paper are: quant-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: q-fin
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The categories of this paper are: physics
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The categories of this paper are: patt-sol
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: alg-geom