The First Billion Years project - IV: Proto-galaxies reionising the Universe

TL;DR

This study uses high-resolution simulations to analyze how proto-galaxies contributed to cosmic reionisation between redshifts 15 and 10, highlighting the dominant role of low-mass galaxies and the limited impact of Population III stars.

Contribution

It provides a self-consistent assessment of galaxy contributions to reionisation, emphasizing the mass and redshift dependence of escape fractions and the limited role of Pop III stars.

Findings

Proto-galaxies in dark-matter haloes of 1e7-1e8 solar masses drove reionisation.

Reionisation occurred mainly between redshifts 15 and 10.

Pop III stars had a minor contribution due to their short lifespans.

Abstract

The contribution of stars in galaxies to cosmic reionisation depends on the star formation history in the Universe, the abundance of galaxies during reionisation, the escape fraction of ionising photons and the clumping factor of the inter-galactic medium (IGM). We compute the star formation rate and clumping factor during reionisation in a cosmological volume using a high-resolution hydrodynamical simulation. We post-process the output with detailed radiative transfer simulations to compute the escape fraction of ionising photons. Together, this gives us the opportunity to assess the contribution of galaxies to reionisation self-consistently. The strong mass and redshift dependence of the escape fraction indicates that reionisation occurred between z=15 and z=10 and was mainly driven by proto-galaxies forming in dark-matter haloes with masses between 1e7 and 1e8 solar mass. More massive galaxies that are rare at these redshifts and have significantly lower escape fractions contribute less photons to the reionisation process than the more-abundant low-mass galaxies. Star formation in the low-mass haloes is suppressed by radiative feedback from reionisation, therefore these proto-galaxies only contribute when the part of the Universe they live in is still neutral. After z~10, massive galaxies become more abundant and provide most of the ionising photons. In addition, we find that Population (Pop) III stars are too short-lived and not frequent enough to have a major contribution to reionisation. Although the stellar component of the proto-galaxies that produce the bulk of ionising photons during reionisation is too faint to be detected by the James Webb Space Telescope (JWST), these sources are brightest in the H-alpha and Ly-alpha recombination lines, which will likely be detected by JWST in deep surveys.