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

The first stars (Pop III stars) and Black Holes (BHs) formed in galaxies at Cosmic Dawn (CD) have not been observed and remain poorly constrained. Theoretical models predict that indirect insights of those Pop III stars and BHs could be imprinted as an absorption signal in the 21cm line of the atomic hydrogen (HI) in the cold Intergalactic Medium (IGM), against the Cosmic Microwave Background (CMB), when the Universe was less than 200 million years old. The first tentative observation of an HI absorption in the 21cm line at redshifts z > 15 by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has stimulated a great deal of research. To explain the additional large amplitude of that absorption signal a plethora of models based on exotic physics and on astrophysical sources have been proposed. Among the latter are models that propose the existence of an additional synchrotron Cosmic Radio Background (CRB) from BH-jet sources of comparable intensity to that of the CMB that boosts the HI absorption signal at CD. The discovery of radio loud supermassive black holes (SMBHs) of ~10⁹ M_⊙ in high-z quasars of up to z ~7.5 suggests the existence of a CRB component 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 would be required. With no judgment on whether the EDGES signal is of cosmic origin or not, here we provide approximate calculations to analyze highly redshifted HI absorption signals taking that of EDGES as an example to explore what could be learned on BHs at CD. Assuming a BH mass to radio luminosity ratio as observed in radio-loud Supermassive BHs (SMBHs) of ∼10⁹ M_⊙ in quasars at redshifts z = 6 - 7, by simple calculations 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 can explain the onset of the EDGES absorption at 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 of Stellar Mass Density (SMD) expected at those high redshifts. This suggests that those IMBHs are formed before, and growing faster than the bulk of stars, with no need of a large mass contribution from stellar-mass BH remnants of typical Pop III stars. The highly redshifted signals from these IMBHs at cosmic dawn may be detected at long radio wavelengths with the next generation of ultrasensitive interferometers such as the Square Kilometer Array (SKA), in the infrared with the James Webb Space Telescope (JWST), and in the X-rays with future space missions.