The evolution of [O <scp>iii</scp>]  + Hβ equivalent width from <i>z</i> ≃ 3−8: implications for the production and escape of ionizing photons during reionization

ABSTRACT Accurately quantifying the ionizing photon production efficiency ($\xi _\mathrm{ion}$) of $z\gtrsim 6$ 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 $z\simeq 6.9-7.6$ 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 ($W_{\lambda }$) of their [O iii] + $\mathrm{H\, \beta }$ emission lines and their physical properties. To supplement this sample, we measure $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$) photometrically for $N=253$$z_{\mathrm{spec}}=3.2-3.6$ SFGs selected from the VANDELS spectroscopic survey. Comparing these samples, we find a strong apparent redshift evolution in their median $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$), increasing from $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$)$=380\pm 30$ Å in VANDELS to $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$)$=540\pm 25$ Å in PRIMER$+$JADES. Concentrating on the JWST sample ($z\gtrsim 7$), we find that $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$) correlates with stellar mass and ultraviolet (UV) luminosity, with high-mass, $M_{\mathrm{UV}}$-faint galaxies producing systematically weaker emission lines. Moreover, we discover a departure from the standard lognormal shape of the $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$) distribution, with a more pronounced tail towards lower $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$), consistent with increasingly bursty star formation. Using $W_{\lambda }$([O iii] + $\mathrm{H\, \beta }$) as a proxy for $\xi _\mathrm{ion}$, and UV spectral slope as a proxy for Lyman-continuum escape ($f_\mathrm{esc}^\mathrm{LyC}$), we uncover a minority of galaxies with high $\xi _\mathrm{ion}$ and $f_\mathrm{esc}^\mathrm{LyC}$ (e.g. $\mathrm{log}(\xi _\mathrm{ion}/\mathrm{erg^{-1}Hz})\simeq 25.6$ and $f_\mathrm{esc}^\mathrm{LyC}$$\simeq 0.15$). However, we find the ionizing photon budget at $z\gtrsim 7$ is dominated by galaxies with more moderate output, close to the median values of $\mathrm{log}(\xi _\mathrm{ion}/\mathrm{erg^{-1}Hz})\simeq 25.3$ and $f_\mathrm{esc}^\mathrm{LyC}$$\simeq 0.05$. Our results are consistent with estimates for the number of ionizing photons required to power reionization at $z\gtrsim 7$, with no evidence for over or underproduction.