Bridging the Gap between Cosmic Dawn and Reionization Favors Models Dominated by Faint Galaxies

It has been claimed that traditional models struggle to explain the tentative detection of the 21 cm absorption trough centered at z similar to 17 measured by the EDGES collaboration. On the other hand, it has been shown that the EDGES results are consistent with an extrapolation of a declining UV luminosity density, following a simple power law of deep Hubble Space Telescope observations of 4 < z < 9 galaxies. We here explore the conditions by which the EDGES detection is consistent with current reionization and post-reionization observations, including the neutral hydrogen fraction at z similar to 6-8, Thomson-scattering optical depth, and ionizing emissivity at z similar to 5. By coupling a physically motivated source model derived from radiative transfer hydrodynamic simulations of reionization to a Markov Chain Monte Carlo sampler, we find that it is entirely possible to reconcile existing high-redshift (cosmic dawn) and low-redshift (reionization) constraints. In particular, we find that high contributions from low-mass halos along with high photon escape fractions are required to simultaneously reproduce cosmic dawn and reionization constraints. Our analysis further confirms that low-mass galaxies produce a flatter emissivity evolution, which leads to an earlier onset of reionization with a gradual and longer duration, resulting in a higher optical depth. While the models dominated by faint galaxies successfully reproduce the measured globally averaged quantities over the first one billion years, they underestimate the late redshift-instantaneous measurements in efficiently star-forming and massive systems. We show that our (simple) physically motivated semianalytical prescription produces results that are consistent with the (sophisticated) state-of-the-art THESAN radiation-magnetohydrodynamic simulation of the reionization.

Automated summary

Traditional models struggle to explain the detection of the 21 cm absorption trough measured by the EDGES collaboration, but the results are consistent with observations of deep space galaxies. This study explores the conditions for the EDGES detection to be consistent with current observations of reionization and post-reionization.