Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2018
- DOI:
- 10.3847/1538-4357/aae51d
- arXiv:
- arXiv:1803.01815
- Bibcode:
- 2018ApJ...868...63E
- Keywords:
-
- dark ages;
- reionization;
- first stars;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 10 pages, 7 figures, accepted to ApJ, replacement to match submitted version