The production and escape of Lyman-Continuum radiation from star-forming galaxies atz∼ 2 and their redshift evolution

We study the production rate of ionizing photons of a sample of 588 Hα emitters (HAEs) and 160 Lyman-α emitters (LAEs) at z = 2. 2 in the COSMOS field in order to assess the implied emissivity from galaxies, based on their ultraviolet (UV) luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/NUV data, we find fesc fesc〉 of 5. 9^+14. 5-₄. ₂ per cent at z = 2. 2 if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global 〈fesc〉 at z 3–5, also ruling out a high 〈fesc〉 at z ion, the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ξion 1024. 77 ± 0. 04 Hz erg−1 for HAEs and ξion 1025. 14 ± 0. 09 Hz erg−1 for LAEs. LAEs and low-mass HAEs at z = 2. 2 show similar values of ξion as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ξion with increasing EW (Hα), ξion likely increases with redshift. This evolution alone is fully in line with the observed evolution of ξion between z 2 and 5, indicating a typical value of ξion 1025. 4 Hz erg−1 in the reionization era.