Constraining structure formation using EDGES
Abstract
The experiment to detect the global epoch of reionization signature (EDGES) collaboration reported the detection of a line at 78MHz in the sky-averaged spectrum due to neutral hydrogen (HI) 21-cm hyperfine absorption of cosmic microwave background photons at z 17. This requires that the spin temperature of HI be coupled to the kinetic temperature of the gas at this z through the scattering of Ly-α photons emitted by massive stars. To explain the experimental result, star formation needs to be sufficiently efficient at z 17 and this can be used to constrain models in which small-scale structure formation is suppressed (DMF models), either due to dark matter free-streaming or non-standard inflationary dynamics. We combine simulations of structure formation with a simple recipe for star formation to investigate whether these models emit enough Ly-α photons to reproduce the experimental signal for reasonable values of the star formation efficiency, f. We find that a thermal warm dark matter (WDM) model with mass mWDM 4.3\,keV is consistent with the timing of the signal for f 2\%. The exponential growth of structure around z 17 in such a model naturally generates a sharp onset of the absorption. A warmer model with mWDM 3\,keV requires a higher star formation efficiency, f 6\%, which is a factor of few above predictions of current star formation models and observations of satellites in the Milky Way. However, uncertainties in the process of star formation at these z do not allow to derive strong constrains on such models using 21-cm absorption line. The onset of the 21-cm absorption is generally faster in DMF compared to cold dark matter (CDM), unless some process significantly suppresses star formation in halos with masses below 108\,h-1\,M.