The birth of a galaxy - III. Propelling reionization with the faintest galaxies

Starlight from galaxies plays a pivotal role throughout the process of cosmic reionization. We present the statistics of dwarf galaxy properties at z > 7 in haloes with masses up to 10⁹ M_⊙, using a cosmological radiation hydrodynamics simulation that follows their buildup starting with their Population III progenitors. We find that metal-enriched star formation is not restricted to atomic cooling (T_vir ≥ 10⁴ K) haloes, but can occur in haloes down to masses ∼10⁶ M_⊙, especially in neutral regions. Even though these smallest galaxies only host up to 10⁴ M_⊙ of stars, they provide nearly 30 per cent of the ionizing photon budget. We find that the galaxy luminosity function flattens above M_UV ∼ -12 with a number density that is unchanged at z ≲ 10. The fraction of ionizing radiation escaping into the intergalactic medium is inversely dependent on halo mass, decreasing from 50 to 5 per cent in the mass range log M/M_⊙ = 7.0-8.5. Using our galaxy statistics in a semi-analytic reionization model, we find a Thomson scattering optical depth consistent with the latest Planck results, while still being consistent with the UV emissivity constraints provided by Lyα forest observations at z = 4-6.