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

Accurately quantifying the ionizing photon production efficiency (ξ_ion) 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 (Public Release IMaging for Extragalactic Research) and JWST Advanced Deep Extragalactic Survey (JADES). We use BAGPIPES to consistently infer the equivalent widths (W_λ) of their [O III] + H β emission lines and their physical properties. To supplement this sample, we measure W_λ([O III] + H β) photometrically for N = 253z_spec = 3.2 − 3.6 SFGs selected from the VANDELS spectroscopic survey. Comparing these samples, we find a strong apparent redshift evolution in their median W_λ([O III] + H β), increasing from W_λ([O III] + H β)= 380 ± 30 Å in VANDELS to W_λ([O III] + H β)= 540 ± 25 Å in PRIMER+JADES. Concentrating on the JWST sample (z ≿ 7), we find that W_λ([O III] + H β) correlates with stellar mass and ultraviolet (UV) luminosity, with high-mass, M_UV-faint galaxies producing systematically weaker emission lines. Moreover, we discover a departure from the standard lognormal shape of the W_λ([O III] + H β) distribution, with a more pronounced tail towards lower W_λ([O III] + H β), consistent with increasingly bursty star formation. Using W_λ([O III] + H β) as a proxy for ξ_ion, and UV spectral slope as a proxy for Lyman-continuum escape (f_esc^LyC), we uncover a minority of galaxies with high ξ_ion and f_esc^LyC (e.g. log(ξ_ion/erg⁻¹Hz) 25.6 and f_esc^LyC 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(ξ_ion/erg⁻¹Hz) 25.3 and f_esc^LyC 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 underproduction.