Efficient Ionizers with Low Hβ + [O iii] Equivalent Widths: JADES Spectroscopy of a Peculiar High-redshift Population

Early JWST photometric studies discovered a population of UV-faint ( < L UV * ) z ∼ 6.5-8 Lyman break galaxies with spectral energy distributions implying young ages (∼10 Myr) yet relatively weak Hβ + [O iii] equivalent widths (EW_{Hβ + [O iii]} ≈ 400 Å). These galaxies seemingly contradict the implicit understanding that young star-forming galaxies are ubiquitously strong Hβ + [O iii] emitters, i.e., extreme emission line galaxies (EW ≳750 Å). Low metallicities, high Lyman continuum escape fractions, and rapidly declining star formation histories have been proposed as primary drivers behind low Hβ + [O iii] EWs, but the blend of Hβ + [O iii] 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. We find that a significant subset of these galaxies at z ≳ 2 with modest Hβ + [O iii] EWs (≈300-600 Å) have high ionization efficiencies ( log ξ_ion≳25.5 Hz erg⁻¹). Suppressed [O iii] EW values yet elevated Hα and Hβ EW values imply that the level of chemical enrichment is the primary culprit, supported by spectroscopic measurements of metallicities below 12 + log(O/H) ≈ 7.70 (0.1Z_⊙). We demonstrate that integrated Hβ + [O iii] selections (e.g., Hβ + [O iii] EW > 700 Å) 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 Hα space, enabling the identification of these metal-poor efficient ionizers by their specific star formation rate.