First light and reionization epoch simulations (FLARES) X III: the lyman-continuum emission of high-redshift galaxies

The history of reionization is highly dependent on the ionizing properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionizing properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionization Epoch Simulations (FLARES) to study the Lyman-continuum (LyC, i.e. hydrogen-ionizing) emission of massive ($M_*\gt 10^8\, \mathrm{M_\odot }$) galaxies at redshifts z = 5 - 10. We find that the specific ionizing emissivity (i.e. intrinsic ionizing emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. FLARES predicts a median ionizing photon production efficiency (i.e. intrinsic ionizing emissivity per unit intrinsic far-UV luminosity) of $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})=25.40^{+0.16}_{-0.17}$, with values spanning the range $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})=25-25.75$. This is within the range of many observational estimates, but below some of the extremes observed. We compare the production efficiency with observable properties, and find a weak negative correlation with the UV-continuum slope, and a positive correlation with the [O III] equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionizing emissivity per unit dust-attenuated far-UV luminosity), and find a median of $\log _{10}(\xi _{\rm ion}\rm {/erg^{-1}Hz})\sim 25.5$. Within our sample of $M_*\gt 10^8\, \mathrm{M_\odot }$ galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionizing emissivity. Active galactic nuclei (AGN) emission accounts for 10 - 20 per cent of the total emissivity at a given redshift, and extends the LyC luminosity function by ~0.5 dex.