Constraining structure formation using EDGES

Matteo Leo, Tom Theuns, Carlton M. Baugh, Baojiu Li, Silvia Pascoli

Published 2019-09-10Version 1

The experiment to detect the global epoch of reionization signature (EDGES) collaboration reported the detection of a line at 78~MHz in the sky-averaged spectrum due to neutral hydrogen (HI) 21-cm hyperfine absorption of cosmic microwave background photons at redshift $z\sim17$. This requires that the spin temperature of HI be coupled to the kinetic temperature of the gas at this redshift through scattering of Lyman-$\alpha$ photons emitted by massive stars. To explain the experimental result, star-formation needs to be sufficiently efficient at $z\sim17$ and this can be used to constrain models in which small-scale structure formation is suppressed (DMF), 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 Lyman-$\alpha$ 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 $m_\mathrm{WDM}\sim4.3\,\mathrm{keV}$ 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 model with $m_\mathrm{WDM}\sim3\,\mathrm{keV}$ requires higher star-formation efficiency, $f_\star\sim6\%$, 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 redshifts do not allow to derive strong constrains on such models using 21-cm absorption line. The onset of the 21-cm absorption is generally slower in DMF than in cold dark matter models, unless some process significantly suppresses star formation in halos with circular velocity below $\sim 20$~km~s$^{-1}$.