Black holes at cosmic dawn in the redshifted 21cm signal of HI

Abstract

Indirect insights of Pop III stars and Black Holes (BHs) at Cosmic Dawn (CD) may be imprinted as an absorption signal in the 21cm line of HI against the CMB, when the Universe was less than 200 Myr old. To explain the additional large amplitude of the 21cm HI absorption reported by EDGES there have been proposed models based on an additional synchrotron Cosmic Radio Background (CRB) from BH-jet sources that boost the HI absorption signal at CD. The recent observations of radio loud supermassive BHs (SMBHs) in high-z quasars up to z=7 suggest the existence of a CRB from growing BHs at z > 15, of unknown intensity. To match the onset of the EDGES signal a CRB of comparable intensity to that of the CMB is required. Here we provide approximate calculations to analyze this type of absorption signals, taking that of EDGES as an example. Assuming a BH mass to radio luminosity ratio as observed in radio-loud SMBHs of ~10^9 solar masses in quasars at z = 6-7, we find that rapidly growing radio luminous BHs of Intermediate Mass (IMBHs) in their way to become SMBHs, are the only type of astrophysical radio sources of a CRB that could explain the amplitude of the HI absorption reported by EDGES in the interval of z = 18-20. At those redshifts the EDGES signal would imply that the global mass density of IMBHs must be dominant over that of stars, more than 70% of the maximum Stellar Mass Density (SMD) expected at those high redshifts. This suggests that those IMBHs are formed before and grow faster than the bulk of stars, with no large mass contribution from stellar-mass BH remnants of typical Pop III stars. The highly redshifted signals from these IMBHs may be detected at long radio wavelengths with ultrasensitive interferometers such as the SKA, in the infrared with the JWST, and in the X-rays with future space missions.