Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Self-shielding Effects on the Column Density Distribution of Damped
  Lyman Alpha Systems   We calculate the column density distribution of damped Lyman alpha systems,
modeled as spherical isothermal gaseous halos ionized by the external cosmic
background. The effects of self-shielding introduce a hump in this
distribution, at a column density N_{HI} \sim 1.6x10^{17} X^{-1} cm^{-2}, where
X is the neutral fraction at the radius where self-shielding starts being
important. The most recent compilation of the column density distribution by
Storrie-Lombardi & Wolfe shows marginal evidence for the detection of this
feature due to self-shielding, suggesting a value X \sim 10^{-3}. Assuming a
photoionization rate \Gamma \sim 10^{-12} s^{-1} from the external ionizing
background, the radius where self-shielding occurs is inferred to be about
3.8kpc. If damped Lyman alpha systems consist of a clumpy medium, this should
be interpreted as the typical size of the gas clumps in the region where they
become self-shielding. Clumps of this size with typical column densities N_H
\sim 3x10^{20} cm^{-2} would be in hydrostatic equilibrium at the
characteristic photoionization temperature \sim 10^4 K if they do not contain
dark matter. Since this size is similar to the overall radius of damped \lya
systems in Cold Dark Matter models, where all halos are assumed to contain
similar gas clouds producing damped absorbers, this suggests that the gas in
damped absorbers is in fact not highly clumped.

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The categories of this paper are: mtrl-th
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The categories of this paper are: q-fin
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
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The categories of this paper are: math-ph
8
The categories of this paper are: hep-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: comp-gas
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The categories of this paper are: supr-con
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-alg
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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: eess
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The categories of this paper are: dg-ga
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The categories of this paper are: physics