Characterizing Extreme Emission-line Galaxies. I. A Four-zone Ionization Model for Very High-ionization Emission*

Abstract Stellar population models produce radiation fields that ionize oxygen up to O +2 , defining the limit of standard H ii region (<54.9 eV). Yet, some extreme emission-line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra two nearby EELGs very high-ionization lines (e.g., He ?? 1640,4686 C iv 1548,1550, [Fe v ] ? 4227, [Ar 4711,4740). define a four-zone model is augmented by zone, as characterized (>54.4 The has little no effect on measured total nebular abundances, but does change interpretation other EELG properties: we measure steeper central gradients; volume-averaged parameters; T e n log U values. Traditional three-zone estimates parameter can underestimate average 0.5 dex. Additionally, find model-independent dichotomy in abundance patterns, where ? /H abundances are consistent N/H, C/H, Fe/H relatively deficient, suggesting these /Fe-enriched more than three times. However, there still high-energy ionizing photon production problem (HEIP 3 ). Even for such /Fe enrichment high s, photoionization cannot reproduce observed EELGs.