Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: THE LYMAN-ALPHA FOREST AT LOW REDSHIFT: TRACING THE DARK MATTER
  FILAMENTS   We study the distribution of low-redshift Ly$\alpha$ clouds in a CDM model
using numerical simulations including photoionization and cooling of the
baryonic component. The ionizing background is found to be efficient enough to
keep most of the gas warm ($T$~=~1--5~10$^4$~K) and to prevent most of it from
collapsing. In this scenario, the numerous low redshift ($z$~$<$~0.5) neutral
hydrogen Ly$\alpha$ lines recently revealed by HST observations of QSOs
originate in the warm gas that traces the potential wells of filamentary
structures defined by the dark matter. The predicted line number density is
consistent with observations. About 25\% of the Ly$\alpha$ lines are found to
arise in extended envelopes of luminous galaxies or groups of galaxies
($M$~$>$~3~10$^{11}$~M$_{\odot}$) of radius of the order of 1.5~Mpc. The exact
number, however, is model dependent. The remainder arises in gas independent of
any galaxy and at a distance of 2 to 7~Mpc from the first neighbour galaxy.

29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
9
The categories of this paper are: nlin
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th
16
The categories of this paper are: stat
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph