The evolution of [O iii]+H 𝛽 equivalent width from z ≃ 3 − 8: implications for the production and escape of ionizing photons during reionization

Accurately quantifying the ionizing photon production efficiency (𝜉ion) of 𝑧 ≳ 6 star-forming galaxies (SFGs) is necessary to fully understand their contribution to reionization. In this study, we investigate the ionizing properties of 𝑁 = 279 SFGs selected at 𝑧 ≃ 6.9−7.6 from two of the largest JWST Cycle-1 imaging programmes; PRIMER and JADES. We use bagpipes to consistently infer the equivalent widths (𝑊𝜆) of their [O iii]+H 𝛽 emission lines and their physical properties. To supplement this sample, we measure 𝑊𝜆([O iii]+H 𝛽) photometrically for 𝑁 = 253 𝑧spec = 3.2 − 3.6 SFGs selected from the VANDELS spectroscopic survey. Comparing these samples, we find a strong apparent redshift evolution in their median 𝑊𝜆([O iii]+H 𝛽), increasing from 𝑊𝜆([O iii]+H 𝛽)= 380 ± 30Å in VANDELS to 𝑊𝜆([O iii]+H 𝛽)= 540 ± 25Å in PRIMER+JADES. Concentrating on the JWST sample (𝑧 ≳ 7), we find that 𝑊𝜆([O iii]+H 𝛽) correlates with stellar mass and UV luminosity, with high-mass, 𝑀UV-faint galaxies producing systematically weaker emission lines. Moreover, we discover a departure from the standard log-normal shape of the 𝑊𝜆([O iii]+H 𝛽) distribution, with a more pronounced tail towards lower 𝑊𝜆([O iii]+H 𝛽), consistent with increasingly bursty star formation. Using 𝑊𝜆([O iii]+H 𝛽) as a proxy for 𝜉ion, and UV spectral slope as a proxy for Lyman-continuum escape ( 𝑓 LyC esc ), we uncover a minority of galaxies with high 𝜉ion and 𝑓 LyC esc (e.g., log(𝜉ion/erg−1Hz) ≃ 25.6 and 𝑓 LyC esc ≃ 0.15). However, we find the ionizing photon budget at 𝑧 ≳ 7 is dominated by galaxies with more moderate output, close to the median values of log(𝜉ion/erg−1Hz) ≃ 25.3 and 𝑓 LyC esc ≃ 0.05. Our results are consistent with estimates for the number of ionizing photons required to power reionization at 𝑧 ≳ 7, with no evidence for over or under-production.