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 HII region models ( 54.4 eV). The 4-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 logU values. Traditional 3-zone estimates of the ionization parameter can under-estimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the alpha/H-abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are alpha/Fe-enriched by >3 times. However, there still is a high-energy ionizing photon production problem (HEIP^3). Even for such alpha/Fe-enrichment and very-high log Us, photoionization models cannot reproduce the very-high-ionization emission lines observed in EELGs.