Astrophysical information from the Rayleigh-Jeans Tail of the CMB

One of the explanations for the recent EDGES-LOW band 21-cm measurements of a strong absorption signal around 80~MHz is the presence of an excess radio background to the Cosmic Microwave Background (CMB). Such excess can be produced by the decay of unstable particles into small mass dark photons which have a non-zero mixing angle with electromagnetism. We use the EDGES-LOW band measurements to derive joint constraints on the properties of the early galaxies and the parameters of such a particle physics model for the excess radio background. A Bayesian analysis shows that a high star formation efficiency and an X-ray luminosity of $1-2 times 10^{41} rm erg ~s^{-1} ~ Mpc^{-3}$ are required along with a suppression of star formation in halos with virial temperatures $lesssim 2times 10^4$ K. The same analysis also suggests a 68 percent credible intervals for the mass of the decaying dark matter particles, its lifetime, dark photon mass and the mixing angle of the dark and ordinary photon oscillation of $[10^{-3.5}, 10^{-2.4}]$ eV, $[10^{1.1}, 10^{2.7}]times tau_U$, $[10^{-12.2}, 10^{-10}]$ eV and $[10^{-7}, 10^{-5.6}]$ respectively. This implies an excess radio background which is $approx 5.7$ times stronger than the CMB around 80~MHz. This value is a factor $sim 3$ higher than the previous predictions which used a simplified model for the 21-cm signal.