Constraints on early star formation from the 21-cm global signal

The tentative detection by the EDGES experiment of a global 21-cm absorption trough centred at redshift 17 opens up the opportunity to study the birth of the first luminous sources, the intensity of radiation backgrounds at cosmic dawn, the thermal and ionization history of the young intergalactic medium. Here, we focus on the astrophysical implications of the Ly α photon field needed to couple the spin temperature to the kinetic temperature of the gas at these early epochs. Under the basic assumption that the 21-cm signal is activated by extremely metal-poor stellar systems, we show that the EDGES results are consistent with an extrapolation of the declining galaxy UV luminosity density measured at $4\lesssim z\lesssim 9$ by deep Hubble Space Telescope observations. A substantially enhanced star formation rate density or new exotic sources of UV photons are not required at the redshifts of the EDGES signal. The amount of ionizing radiation produced by the same stellar systems that induce Ly α coupling is significant, of order 0.5 LyC photons per H-atom per 100 Myr. To keep hydrogen largely neutral and delay the reionization process consistently with recent Planck cosmic microwave background results, mean escape fractions of $f_{\rm esc}\lesssim 20$ per cent are required at $z$ > 15.