2017MNRAS.465.3637M

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 f_esc < 2.8 (6.4) per cent through median (mean) stacking. By combining the Hα luminosity density with intergalactic medium emissivity measurements from absorption studies, we find a globally averaged <f_esc> 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 <f_esc> at z ≃ 3-5, also ruling out a high <f_esc> at z < 5. These low escape fractions allow us to measure ξ_ion, the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ξ_ion ≃ 10^24.77 ± 0.04^ Hz erg^–1 for HAEs and ξ_ion ≃ 10^25.14 ± 0.09^ Hz erg^–1 for LAEs. LAEs and low-mass HAEs at z = 2.2 show similar values of ξ_ion as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ξ_ion with increasing EW(Hα), ξ_ion likely increases with redshift. This evolution alone is fully in line with the observed evolution of ξ_ion between z ≃ 2 and 5, indicating a typical value of ξ_ion ≃ 10^25.4 Hz erg^–1 in the reionization era.