Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude

摘要

We estimate the 21 cm radio background from accretion onto the first intermediate-mass black holes between z?≈?30 and z?≈?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?≈?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 ≈10?13–10?12 erg s?1 cm?2 deg?2, consistent with existing constraints. In order to avoid heating the intergalactic medium (IGM) over the EDGES trough, these black holes would need to be obscured by hydrogen column depths of N H?~?5?×?1023 cm?2. Such black holes would avoid violating constraints on the CMB optical depth from Planck if their UV photon escape fractions were below f esc??0.1, which would be a natural result of N H?~?5?×?1023 cm?2 being imposed by an unheated IGM.