Theoretical modelling of emission-line galaxies: new classification parameters for mid-infrared and optical spectroscopy

We have carried out extensive and detailed photoionization modelling to successfully constrain the locations of different emission-line galaxies in optical and mid-infrared diagnostic diagrams. Our model grids cover a wide range in parameter space for the active galaxy continuum and starburst galaxies with different stellar population laws and metallicities. We compare the predicted AGN and star formation mid-infrared line ratios [Ne iii]15.56 μm/[Ne ii]12.81 μm and [O iv]25.89 μm/[Ne iii]15.56 μm to the observed values, and find that the best fit for the active galactic nucleus (AGN) is via a two-zone approximation. This two-zone approximation is a combination of a matter-bounded component, where [Ne iii] and [O iv] are emitted efficiently, and a radiation-bounded component that maximizes [Ne ii] emission. We overlay the predictions from this two-zone approximation on to the optical [O iii]λ5007/Hβ and [N ii]λ6583/Hα diagnostic diagram derived from the Sloan Digital Sky Survey, to find that the high-density and low-ionization radiation-bounded component in our two-zone AGN approximation model provides a good lower limit for [N ii] emission. This establishes a new theoretical demarcation line for the minimum AGN contribution in this diagram. This new classification results by a factor of ∼1.4 in a higher AGN population than predictions derived from previous divisions of star-forming galaxies. Similarly, we define a maximum AGN contribution in the [O iii]/Hβ and [N ii]/Hα diagram by using a two-zone approximation within a parameter range typical of the narrow-line region.