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

TL;DR

This paper develops detailed photoionization models to improve the classification of emission-line galaxies, especially AGNs, using optical and mid-infrared diagnostic diagrams, introducing new theoretical demarcation lines.

Contribution

It presents a novel two-zone approximation model for AGN emission, refining galaxy classification and increasing the identified AGN population by approximately 40%.

Findings

Two-zone model effectively fits mid-infrared line ratios.

New demarcation line increases AGN classification by 1.4 times.

Model provides lower and upper limits for AGN contribution.

Abstract

We have carried out extensive and detailed photoionization modeling 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]15.56mm/[Ne II]12.81mm and [O IV]25.89mm/[Ne III]15.56mm to the observed values, and find that the best fit for the 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 onto the optical [O III]l5007/Hbeta and [N II]l6583/Halpha 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]/Hbeta and [N II]/Halpha diagram by using a two-zone approximation within a parameter range typical of the narrow-line region.