Galactic ionising photon budget during the Epoch of Reionisation in the Cosmic Dawn II simulation
Abstract
Cosmic Dawn ("CoDa") II yields the first statistically-meaningful determination of the relative contribution to reionization by galaxies of different halo mass, from a fully-coupled radiation-hydrodynamics simulation of the epoch of reionization large enough ( 100 Mpc) to model global reionization while resolving the formation of all galactic halos above 108 M. Cell transmission inside high-mass haloes is bi-modal -- ionized cells are transparent, while neutral cells absorb the photons their stars produce - and the halo escape fraction fesc reflects the balance of star formation rate ("SFR") between these modes. The latter is increasingly prevalent at higher halo mass, driving down fesc (we provide analytical fits to our results), whereas halo escape luminosity, proportional to fesc ×SFR, increases with mass. Haloes with dark matter masses within 6.108 M < Mh < 3.1010 M produce 80% of the escaping photons at z=7, when the Universe is 50% ionized, making them the main drivers of cosmic reionization. Less massive haloes, though more numerous, have low SFRs and contribute less than 10% of the photon budget then, despite their high fesc. High mass haloes are too few and too opaque, contributing <10% despite their high SFRs. The dominant mass range is lower (higher) at higher (lower) redshift, as mass function and reionization advance together (e.g. at z=8.5, x HI=0.9, Mh < 5.109 M haloes contributed 80%). Galaxies with UV magnitudes MAB1600 between -12 and -19 dominated reionization between z=6 and 8.