First Light And Reionisation Epoch Simulations (FLARES) XIII: The Lyman-continuum emission of high-redshift galaxies

TL;DR

This study uses FLARES simulations to analyze the Lyman-continuum emission of high-redshift galaxies, revealing how ionising properties relate to galaxy mass, age, metallicity, and observable features, with implications for cosmic reionisation.

Contribution

The paper provides new insights into the ionising photon production efficiency of high-redshift galaxies and its dependence on physical and observable galaxy properties, based on detailed simulations.

Findings

Ionising emissivity decreases with increasing stellar mass.

Median ionising photon production efficiency is around 25.4 in log scale.

Active galactic nuclei contribute 10-20% to total ionising emissivity.

Abstract

The history of reionisation is highly dependent on the ionising properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionising properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionisation Epoch Simulations (FLARES) to study the Lyman-continuum (LyC, i.e. hydrogen-ionising) emission of massive ($M_*>10^8\,\mathrm{M_\odot}$) galaxies at redshifts $z=5-10$. We find that the specific ionising emissivity (i.e. intrinsic ionising emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. FLARES predicts a median ionising photon production efficiency (i.e. intrinsic ionising 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 OIII equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionising emissivity per unit dust-attenuated far-UV luminosity), and find a median of $\log_{10}(\xi_{\rm ion}\rm{/erg^{-1}Hz})\sim25.5$. Within our sample of $M_*>10^8\,\mathrm{M_\odot}$ galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionising emissivity. Active galactic nuclei (AGN) emission accounts for $10-20$ % of the total emissivity at a given redshift, and extends the LyC luminosity function by $\sim0.5$ dex.