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We have carried out extensive and detailed photoionization modeling tosuccessfully constrain the locations of different emission-line galaxies inoptical and mid-infrared diagnostic diagrams. Our model grids cover a widerange in parameter space for the active galaxy continuum and starburst galaxieswith different stellar population laws and metallicities. We compare thepredicted AGN and star-formation mid-infrared line ratios [Ne III]15.56mm/[NeII]12.81mm and [O IV]25.89mm/[Ne III]15.56mm to the observed values, and findthat the best fit for the AGN is via a two-zone approximation. This two-zoneapproximation is a combination of a matter-bounded component, where [Ne III]and [O IV] are emitted efficiently, and a radiation-bounded component thatmaximizes [Ne II] emission. We overlay the predictions from this two-zoneapproximation onto the optical [O III]l5007/Hbeta and [N II]l6583/Halphadiagnostic diagram derived from the Sloan Digital Sky Survey, to find that thehigh-density and low-ionization radiation-bounded component in our two-zone AGNapproximation model provides a good lower limit for [N II] emission. Thisestablishes a new theoretical demarcation line for the minimum AGN contributionin this diagram. This new classification results by a factor of ~1.4 in ahigher AGN population than predictions derived from previous divisions ofstar-forming galaxies. Similarly, we define a maximum AGN contribution in the[O III]/Hbeta and [N II]/Halpha diagram by using a two-zone approximationwithin a parameter range typical of the narrow-line region.

Theoretical Modeling of Emission-Line galaxies: New Classification Parameters for Mid-Infrared and Optical Spectroscopy