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 $z\sim 17$. 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-$\alpha$ photons emitted by massive stars. To explain the experimental result, star formation needs to be sufficiently efficient at $z\sim 17$ 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-$\alpha$ photons to reproduce the experimental signal for reasonable values of the star formation efficiency, $f_\star$. We find that a thermal warm dark matter (WDM) model with mass $m_\mathrm{WDM}\sim 4.3\,\mathrm{keV}$ is consistent with the timing of the signal for $f_\star\lesssim 2\%$. The exponential growth of structure around $z\sim 17$ in such a model naturally generates a sharp onset of the absorption. A warmer model with $m_\mathrm{WDM}\sim 3\,\mathrm{keV}$ requires a higher star formation efficiency, $f_\star\sim 6\%$, 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 $\sim 10^8\,h^{-1}\,\mathrm{M}_\odot$.