A hint on the metal-free star formation rate density from 21cm-EDGES
  data
  We aim to provide the first data-constrained estimate of the metal-free
(Population III; Pop III) star formation rate density $\dot{\rho}_{*}^{III}$
required at high-redshifts ($z \gtrsim 16$) in order to reproduce both the
amplitude and the redshift of the EDGES 21-cm global signal. Our model accounts
for the Lyman Alpha (Ly$\alpha$), radio and X-ray backgrounds from both Pop III
and metal-enriched Population II (Pop II) stars. For the latter, we use the
star formation rate density estimates (and the Ly$\alpha$ background) from the
{\it Delphi} semi-analytic model that has been shown to reproduce all key
observables for galaxies at $z \gtrsim 5$; the radio and X-ray backgrounds are
fixed using low-$z$ values. The constraints on the free parameters
characterizing the properties of the Pop III stars are obtained using a Markov
Chain Monte Carlo analysis. Our results yield a $\dot{\rho}_{*}^{III}$ that
whilst increasing from $z \sim 21-16$ thereafter shows a sharp decline which is
in excellent agreement with the results found by \citet{valiante2016} to
simulate the growth of $z \sim 6 - 7$ quasars and their host galaxies,
suggesting that the bulk of Pop III star formation occurs in the rarest and
most massive metal-poor halos at $z \lesssim 20$. This allows Pop III stars to
produce a rapidly growing Ly$\alpha$ background between $z \sim 21-15$.
Further, Pop III stars are required to provide a radio background that is about
$3-4$ orders of magnitude higher than that provided by Pop II stars although
Pop II stars dominate the X-ray background.
