Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Descending from on high: Lyman series cascades and spin-kinetic
  temperature coupling in the 21 cm line   We examine the effect of Lyman continuum photons on the 21 cm background in
the high-redshift universe. The brightness temperature of this transition is
determined by the spin temperature T_s, which describes the relative
populations of the singlet and triplet hyperfine states. Once the first
luminous sources appear, T_s is set by the Wouthuysen-Field effect, in which
Lyman-series photons mix the hyperfine levels. Here we consider coupling
through n>2 Lyman photons. We first show that coupling (and heating) from
scattering of Lyman-n photons is negligible, because they rapidly cascade to
lower-energy photons. These cascades can result in either a Lyman-alpha photon
-- which will then affect T_s according to the usual Wouthuysen-Field mechanism
-- or photons from the 2s -> 1s continuum, which escape without scattering. We
show that a proper treatment of the cascades delays the onset of strong
Wouthuysen-Field coupling and affects the power spectrum of brightness
fluctuations when the overall coupling is still relatively weak (i.e., around
the time of the first stars). Cascades damp fluctuations on small scales
because only ~ 1/3 of Lyn photons cascade through Lyman-alpha, but they do not
affect the large-scale power because that arises from those photons that
redshift directly into the Lyman-alpha transition. We also comment on the
utility of Lyman-n transitions in providing "standard rulers" with which to
study the high-redshift universe.

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The categories of this paper are: acc-phys
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The categories of this paper are: eess
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
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The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
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The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: quant-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-ex
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The categories of this paper are: cs
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The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
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The categories of this paper are: chem-ph