IRAC-Excess Galaxies in the NIRSpec Wide Survey: physical properties of bright EELGs at $z\sim7$ and implications for Reionization

Abstract

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]$+\rm{H}\beta$ EW $=1583$~{\AA}, consistent with predictions from the pre-JWST selection and we identify $17/23$ $(\sim74\%)$ of our sample as extreme emission line galaxies (EELGs), based on a selection of [OIII]$+\rm{H}\beta$ EW $\geq1000$~{\AA}. 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 ($\langle M_\mathrm{UV}\rangle=-20.7$) EELG sample is characterised by sub-solar metallicities $(7.38<\rm 12 + \log(O/H)<8.12)$, young stellar populations ($t_\star \sim 20$ Myr), and enhanced ionizing photon production efficiencies, with median $\log_{10}(\xi_\mathrm{ion}/\mathrm{erg}^{-1}~\mathrm{Hz}) = 25.62$. We find that more UV luminous ($\sim L_\mathrm{UV, z=7}^\ast$) EELGs exhibit systematically lower O32 ratios than fainter ($\sim 0.1\times L_\mathrm{UV}^\ast$) EELGs at fixed [OIII]$+\rm{H}\beta$ 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.