The production of ionizing photons in UV-faint z ∼ 3–7 galaxies

Abstract

Aims. The demographics of the production and escape of ionizing photons from UV-faint early galaxies is a key unknown that has hindered attempts to discover the primary drivers of reionization. With the advent of JWST, it is finally possible to observe the rest-frame optical nebular emission from individual sub-L*z > 3 galaxies to measure the production rate of ionizing photons, ξion. Methods. Here we study a sample of 370 z ∼ 3 − 7 galaxies spanning −23 < MUV < −15.5 (median MUV ≈ −18) with deep multiband HST and JWST/NIRCam photometry that covers the rest-UV to the optical from the GLASS and UNCOVER JWST surveys. Our sample includes 102 galaxies with Lyman-alpha emission detected in MUSE spectroscopy. We used Hα fluxes inferred from NIRCam photometry to estimate the production rate of ionizing photons that do not escape these galaxies, ξion(1 − fesc). Results. We find median log10ξion(1 − fesc) = 25.33 ± 0.47, with a broad intrinsic scatter of 0.42 dex, which implies a broad range of galaxy properties and ages in our UV-faint sample. Galaxies detected with Lyman-alpha have ∼0.1 dex higher ξion(1 − fesc), which is explained by their higher Hα equivalent width distribution; this implies younger ages and higher specific star formation rates and, thus, more O/B stars. We find significant trends of increasing ξion(1 − fesc) with increasing Hα equivalent width, decreasing UV luminosity, and decreasing UV slope; this implies that the production of ionizing photons is enhanced in young galaxies with assumed low metallicities. We find no significant evidence for sources with very high ionizing escape fractions (fesc > 0.5) in our sample based on their photometric properties, even amongst the Lyman-alpha-selected galaxies. Conclusions. This work demonstrates that considering the full distribution of ξion across galaxy properties is important for assessing the primary drivers of reionization.