Efficient Ionizers with Low H${\hat{\mathbf{I}}}^2$+[OIII] Equivalent Widths: JADES Spectroscopy of a Peculiar High-z Population

Abstract

Early JWST photometric studies discovered a population of UV faint ($<{\mathrm{L}}_{\mathrm{UV}}^{\ast }$) $z\sim 6.5-8$ Lyman break galaxies with spectral energy distributions implying young ages ($\sim 10$ Myr) yet relatively weak H${\hat{I}}^2$+[OIII] equivalent widths ($\mathrm{E}{\mathrm{W}}_{\mathrm{H}{\hat{\mathrm{I}}}^2+[\mathrm{OIII}]}\approx 400$Ã ). These galaxies seemingly contradict the implicit understanding that young star-forming galaxies are ubiquitously strong H${\hat{I}}^2$+[OIII] emitters, i.e., extreme emission line galaxies (EW $\gtrsim 750$Ã ). Low metallicities, high Lyman continuum escape fractions, and rapidly declining star-formation histories have been proposed as primary drivers behind low H${\hat{I}}^2$+[OIII] equivalent widths, but the blend of H${\hat{I}}^2$+[OIII] in photometric studies makes proving one of these scenarios difficult. We aim to characterize this peculiar population with deep spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES). We find that a significant subset of these galaxies at $z\gtrsim 2$ with modest H${\hat{I}}^2$+[OIII] equivalent widths ($\approx 300-600$Ã ) have high ionization efficiencies ($\log \hat{\mathrm{I}}{¾}_{\mathrm{ion}}\gtrsim 25.5[\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1}]$). Suppressed [OIII] EW values yet elevated H$\hat{I}\pm$ and H${\hat{I}}^2$ EW values imply that the level of chemical enrichment is the primary culprit, supported by spectroscopic measurements of metallicities below 12+log(O/H)$\approx 7.70(10\%{\mathrm{Z}}_{\odot })$. We demonstrate that integrated H${\hat{I}}^2$+[OIII] selections (e.g., H${\hat{I}}^2$+[OIII] EW $>700$Ã ) exclude the most metal-poor efficient ionizers and favor 1) more chemically enriched systems with comparable extreme radiation fields and 2) older starbursting systems. In contrast, metallicity degeneracies are reduced in H$\hat{I}\pm$ space, enabling the identification of these metal-poor efficient ionizers by their specific star-formation rate.

16+4 pages, 7 figures