Strong [OIII]$+$H${\hat{I}}^2$ emitters dominated the ionizing budget at $z\sim 7$

Abstract

We quantify the ionizing photon production at $z\sim 7$ using the deepest spectroscopically confirmed sample of strong [OIII]$+$H${\hat{I}}^2$ emitters (rest-frame EW$>740$ A) in the Abell 2744 field. Leveraging ultra-deep UNCOVER F410M imaging ($5\ddot{I}ƒ\sim 29$ AB) and gravitational lensing, we probe an order of magnitude deeper than previous JWST WFSS [OIII] studies, reaching a luminosity limit of $\log (L_{[\mathrm{OIII}]+\mathrm{H}{\hat{\mathrm{I}}}^2}/{\text{erg s}}^{-1})=41.3$. Our rest-frame optical emission-line selection probes some of the youngest, metal- and dust-poor galaxies, identifying a large population of continuum-faint, ionizing candidates. NIRSpec follow-up of a luminosity-representative subset confirms $72\%$ of targets, providing detailed characterization of 18 emitters. Balmer decrements reveal negligible dust, while strong-line diagnostics indicate extremely low metallicities ($12+\log (\text{O/H})=6.8\text{–}7.7$). With typical [OII]/[OIII] ratios of $0.054\pm 0.007$, we infer an average Lyman continuum escape fraction near the canonical $f_{\text{esc}}=20\%$. Correcting for the spectroscopic confirmation rate, we find that these high-EW emitters represent $56\pm 12\%$ of the total UV-selected population by number density. Integrated to our survey limits, the ionizing budget of these emitters ($\log ({\dot{N}}_{\mathrm{ion}}/{\mathrm{s}}^{-1}{\text{Mpc}}^{-3})=50.63\pm 0.05$) accounts for $\sim 70\%$ of the total budget required for reionization at $z\sim 7$. This result is consistent with empirical benchmarks. These results establish [OIII]$+$H${\hat{I}}^2$ selection as a powerful, dust-insensitive probe, showing that known galaxy populations significantly power reionization.

15 pages, 10 figures, 3 tables; submitted to ApJ