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

Stellar population models produce radiation fields that ionize oxygen up to O⁺², defining the limit of standard H II region models (<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 of two nearby EELGs that have very high-ionization emission lines (e.g., He II λλ1640,4686 C IV λλ1548,1550, [Fe V]λ4227, [Ar IV]λλ4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He⁺² (>54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central T _e , n _e , and log U values. Traditional three-zone estimates of the ionization parameter can underestimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the α/H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are α/Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP³). Even for such α/Fe enrichment and very high log U s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs. *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.