IRAC-Excess Galaxies in the NIRSpec Wide Survey: physical properties of bright EELGs at z about7 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] + β EW =1583 Å, consistent with predictions from the pre-JWST selection and we identify 17/23 (about74%) of our sample as extreme emission line galaxies (EELGs), based on a selection of [OIII] + β EW at least1000 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 UV=-20.7) EELG sample is characterised by sub-solar metallicities (7.38 < 12 + (O/H) < 8.12), young stellar populations (t about 20 Myr), and enhanced ionizing photon production efficiencies, with median 10(ξ ion/erg-1 Hz) = 25.62. We find that more UV luminous (about L UV, z=7) EELGs exhibit systematically lower O32 ratios than fainter (about 0.1 times L UV) EELGs at fixed [OIII] + β 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- α escape fractions (<2 %), suggesting these systems reside in relatively small (0.25 pMpc) ionized regions of the intergalactic medium (IGM), possibly suggesting that bright galaxies have a minor contribution to reionization.