The role of Pop III stars and early black holes in the 21-cm signal from Cosmic Dawn

Modeling the 21-cm global signal from the Cosmic Dawn is challenging due to the many poorly constrained physical processes that come into play. We address this problem using the semi-analytical code 'Cosmic Archaeology Tool' (cat). cat follows the evolution of dark matter haloes tracking their merger history and provides an ab initio description of their baryonic evolution, starting from the formation of the first (Pop III) stars and black holes (BHs) in mini-haloes at z > 20. The model is anchored to observations of galaxies and AGN at z < 6 and predicts a reionization history consistent with constraints. In this work, we compute the evolution of the mean global 21-cm signal between 4 <= z <= 40 based on the rate of formation and emission properties of stars and accreting BHs. We obtain an absorption profile with a maximum depth delta T-b = -95 mK at z similar to 26.5 (54 MHz). This feature is quickly suppressed turning into an emission signal at z = 20 due to the contribution of accreting BHs that efficiently heat the intergalactic medium (IGM) at z < 27. The high-z absorption feature is caused by the early coupling between the spin and kinetic temperature of the IGM induced by Pop III star formation episodes in mini-haloes. Once we account for an additional radio background from early BHs, we are able to reproduce the timing and the depth of the EDGES signal only if we consider a smaller X-ray background from accreting BHs, but not the shape.