Thermal and reionization history within a large-volume semi-analytic galaxy formation simulation

Abstract

We predict the 21-cm global signal and power spectra during the Epoch of Reionization using the MERAXES semi-analytic galaxy formation and reionization model, updated to include X-ray heating and thermal evolution of the intergalactic medium. Studying the formation and evolution of galaxies together with the reionization of cosmic hydrogen using semi-analytic models (such as MERAXES) requires N-body simulations within large volumes and high-mass resolutions. For this, we use a simulation of sidelength 210 h−1 Mpc with 43203 particles resolving dark matter haloes to masses of 5 × 108 h−1 M☉. To reach the mass resolution of atomically cooled galaxies, thought to be the dominant population contributing to reionization, at z = 20 of ∼ 2 × 107 h−1 M☉, we augment this simulation using the DARKFOREST Monte Carlo merger tree algorithm (achieving an effective particle count of ∼1012). Using this augmented simulation, we explore the impact of mass resolution on the predicted reionization history as well as the impact of X-ray heating on the 21-cm global signal and the 21-cm power spectra. We also explore the cosmic variance of 21-cm statistics within 703 h−3 Mpc3 sub-volumes. We find that the midpoint of reionization varies by Δz ∼ 0.8 and that the cosmic variance on the power spectrum is underestimated by a factor of 2–4 at k ∼ 0.1–0.4 Mpc−1 due to the non-Gaussian nature of the 21-cm signal. To our knowledge, this work represents the first model of both reionization and galaxy formation which resolves low-mass atomically cooled galaxies while simultaneously sampling sufficiently large scales necessary for exploring the effects of X-rays in the early Universe.