Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observations of HI 21cm absorption by the neutral IGM during the epoch
  of re-ionization with the Square Kilometer Array   We explore the possibility of detecting HI 21cm absorption by the neutral
intergalactic medium (IGM) toward very high redshift radio sources, and by gas
associated with the first collapsed structures, using the Square Kilometer
Array at low frequency (100 to 200 MHz). The epoch considered is between the
time when the first ionizing sources form and when the bulk of the neutral IGM
becomes ionized. Expected IGM absorption signal includes $\sim 1%$ absorption
by the mean neutral IGM (the radio 'Gunn-Peterson' effect'), plus deeper,
narrow lines ($\ge 5%$, a few km s$^{-1}$) arising in mild density
inhomogeneities with typical values of cosmic overdensity $\delta\sim 10$,
precisely the structures that at later times give rise to the Ly$\alpha$ forest
(the `21cm forest'). Absorption can also arise in gas associated with collapsed
structures ($\delta\ge 100$), including 'minihalos' ($\le 10^7$ M$_\odot$) and
protodisks ($\ge 10^8$ M$_\odot$). We consider SKA sensitivity limits and the
evolution of radio source populations, and conclude that it is reasonable to
hypothesize the existence of an adequate number of high-$z$ radio sources
against which such absorption studies could be performed, provided that
reionization occurs at $z < 10$. Lastly, we discuss the possibility of `line
confusion' due to radio recombination lines arising in the ionized IGM.

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The categories of this paper are: dg-ga
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: supr-con
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The categories of this paper are: eess
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The categories of this paper are: comp-gas
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The categories of this paper are: gr-qc
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: bayes-an
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The categories of this paper are: q-bio
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The categories of this paper are: nucl-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: patt-sol
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The categories of this paper are: adap-org
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The categories of this paper are: math-ph