Modeling the Radio Background from the First Black Holes at Cosmic Dawn:
  Implications for the 21 cm Absorption Amplitude
  We estimate the 21 cm Radio Background from accretion onto the first
intermediate-mass Black Holes between $z\approx 30$ and $z\approx 16$.
Combining potentially optimistic, but plausible, scenarios for black hole
formation and growth with empirical correlations between luminosity and
radio-emission observed in low-redshift active galactic nuclei, we find that a
model of black holes forming in molecular cooling halos is able to produce a 21
cm background that exceeds the Cosmic Microwave Background (CMB) at $z \approx
17$ though models involving larger halo masses are not entirely excluded. Such
a background could explain the surprisingly large amplitude of the 21 cm
absorption feature recently reported by the EDGES collaboration. Such black
holes would also produce significant X-ray emission and contribute to the
$0.5-2$ keV soft X-ray background at the level of $\approx 10^{-13}-10^{-12}$
erg sec$^{-1}$ cm$^{-2}$ deg$^{-2}$, consistent with existing constraints. In
order to avoid heating the IGM over the EDGES trough, these black holes would
need to be obscured by Hydrogen column depths of $ N_\text{H} \sim 5 \times
10^{23} \text{cm}^{-2}$. Such black holes would avoid violating contraints on
the CMB optical depth from Planck if their UV photon escape fractions were
below $f_{\text{esc}} \lesssim 0.1$, which would be a natural result of
$N_\text{H} \sim 5 \times 10^{23} \text{cm}^{-2}$ imposed by an unheated IGM.
