Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radiative Transfer in a Clumpy Universe: III. The Nature of Cosmological
  Ionizing Sources   The history of the transition from a neutral intergalactic medium (IGM) to
one that is almost fully ionized can reveal the character of cosmological
ionizing sources. We study the evolution of the volume filling factor of HII
and HeIII regions in a clumpy IGM, and discuss the implications for rival
reionization scenarios of the rapid decline in the space density of radio-loud
quasars and of the large population of star-forming galaxies recently observed
at z>3. The hydrogen component in a highly inhomogeneous universe is completely
reionized when the number of photons emitted above 1 ryd in one recombination
time equals the mean number of hydrogen atoms. If stellar sources are
responsible for keeping the IGM ionized at z=5, the rate of star formation at
this epoch must be comparable or greater than the one inferred from optical
observations of galaxies at z=3, and the mean metallicity per baryon in the
universe of order 1/500 solar. An early generation of stars in dark matter
halos with circular velocities v_circ=50 km/s, possibly one of the main source
of UV photons at high-z, could be detectable with the Next Generation Space
Telescope. Models in which the quasar emissivity declines rapidly at z>3
predict a late HeII reionization epoch, a feature that could explain the recent
detection of patchy HeII Lyman-alpha at z=2.9 by Reimers et al. (1997) and the
abrupt change observed by Songaila (1998) at about the same epoch of the
SiIV/CIV ratio, but appear unable to provide the required number of
hydrogen-ionizing photons at z=5.

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The categories of this paper are: bayes-an
10
The categories of this paper are: q-bio
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
11
The categories of this paper are: hep-lat
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
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The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
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The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
1
The categories of this paper are: cs
2
The categories of this paper are: math