Fig. 8

image

Comparison of the expected ratios of [O III] and C III] to Hβ fluxes from models of star formation, AGN, and shock excitation, color coded by metallicity, compared to our measurements from the combination of MUSE and G141 spectroscopy. The star-forming grid shows the fiducial model from Jaskot & Ravindranath (2016), the AGN grid shows the dust-free isochoric narrow line region models from Groves et al. (2004), and the shock grid indicates the fully radiative shock plus precursor model from Allen et al. (2008): see the text for more details. The units of (shock) velocity indicate km s-1 and U indicates the dimensionless ionization parameter, volume averaged for the SF grids and the value at the inner edge of the gaseous nebula for the AGN grids. All metallicity values are expressed as a fraction of the solar metallicity. The two vectors show the effect of dust reddening with an AV of 0.038 (the median of our C III] emitters) and 0.5 mag. While dust attenuation is a contributing factor, in general star formation models do not reproduce the observed distribution of line ratios, namely the high [O III]/Hβ ratios in objects with low C III]/Hβ ratios; such a large offset could imply that either more extreme photoionization models, different nebular parameters (such as a lower C/O ratio), or nonstellar forms of excitation are required.

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