Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Implications of a Pre-reionization 21 cm Absorption Signal for Fuzzy
  Dark Matter   The EDGES experiment recently announced evidence for a broad absorption
feature in the sky-averaged radio spectrum around 78 MHz, as may result from
absorption in the 21 cm line by neutral hydrogen at z~15-20. If confirmed, one
implication is that the spin temperature of the 21 cm line is coupled to the
gas temperature by z=20. The known mechanism for accomplishing this is the
Wouthuysen-Field effect, whereby Lyman-alpha photons scatter in the
intergalactic medium (IGM) and impact the hyperfine level populations. This
suggests that early star formation had already produced a copious Lyman-alpha
background by z=20, and strongly constrains models in which the linear matter
power spectrum is suppressed on small-scales, since halo and star formation are
delayed in such scenarios. Here we consider the case that the dark matter
consists of ultra-light axions with macroscopic de Broglie wavelengths (fuzzy
dark matter, FDM). We assume that star formation tracks halo formation and
adopt two simple models from the current literature for the halo mass function
in FDM. We further suppose that the fraction of halo baryons which form stars
is less than a conservative upper limit of $f_\star \leq 0.05$, and that ~10^4
Lyman-alpha to Lyman-limit photons are produced per stellar baryon. We find
that the requirement that the 21 cm spin temperature is coupled to the gas
temperature by $z=20$ places a lower-limit on the FDM particle mass of $m_a
\geq 5 \times 10^{-21} {\rm eV}$. The constraint is insensitive to the precise
minimum mass of halos where stars form. As the global 21 cm measurements are
refined, the coupling redshift could change and we quantify how the FDM
constraint would be modified. A rough translation of the FDM mass bound to a
thermal relic warm dark matter (WDM) mass bound is also provided.

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The categories of this paper are: patt-sol
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The categories of this paper are: econ
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-th
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The categories of this paper are: comp-gas
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: math
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The categories of this paper are: funct-an
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The categories of this paper are: atom-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ph
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The categories of this paper are: stat