IRAC-Excess Galaxies in the NIRSpec Wide Survey: physical properties of bright EELGs at $z\sim 7$ and implications for Reionization

We present an analysis of 23 Spitzer/IRAC excess galaxies at $5.66<z<7.64$, observed as part of the NIRSpec Wide GTO survey. With NIRSpec PRISM spectroscopy, we measure a median rest-frame [OIII]$+\mathrm{H}\beta$ EW $=1583$~Å, consistent with predictions from the pre-JWST selection and we identify $17\mathrm{/}23$ $(\sim 74\mathrm{\%})$ of our sample as extreme emission line galaxies (EELGs), based on a selection of [OIII]$+\mathrm{H}\beta$ EW $\ge 1000$~Å. The typical EELG is powered by star-formation rather than active galactic nuclei (AGN) based on their line widths, line ratios and absence of high-ionization lines, though we cannot exclude an AGN contribution in a few individual sources. We find that our luminous ($⟨M_{\mathrm{U}\mathrm{V}}⟩=-20.7$) EELG sample is characterised by sub-solar metallicities $(7.38<12+\log (\mathrm{O}\mathrm{/}\mathrm{H})<8.12)$, young stellar populations ($t_{\star }\sim 20$ Myr), and enhanced ionizing photon production efficiencies, with median ${\log }_{10}({\xi }_{\mathrm{i}\mathrm{o}\mathrm{n}}\mathrm{/}{\mathrm{e}\mathrm{r}\mathrm{g}}^{-1}\mathrm{H}\mathrm{z})=25.62$. We find that more UV luminous ($\sim L_{\mathrm{U}\mathrm{V},\mathrm{z}=7}^{\ast }$) EELGs exhibit systematically lower O32 ratios than fainter ($\sim 0.1\times L_{\mathrm{U}\mathrm{V}}^{\ast }$) EELGs at fixed [OIII]$+\mathrm{H}\beta$ EW, indicating less extreme radiation fields and/or lower gas densities than their fainter counterparts. Furthermore, bright EELGs exhibit low neutral hydrogen column densities, consistent with a scenario in which they have largely ionized their circumgalactic medium (CGM). Despite this, we find very low Lyman-$\alpha$ escape fractions ($<2$%), suggesting these systems reside in relatively small ($\lesssim 0.25$~pMpc) ionized regions of the intergalactic medium (IGM), possibly suggesting that bright galaxies have a minor contribution to reionization.