  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.
