Star Forming Galaxies as AGN Imposters? A Theoretical Investigation of the Mid-infrared Colors of AGNs and Extreme Starbursts

TL;DR

This study models the mid-infrared spectral energy distributions of dust heated by AGNs and extreme starbursts, revealing how starbursts can mimic AGN colors and proposing new color criteria to distinguish them.

Contribution

It provides the first theoretical models of mid-infrared colors for AGNs and starbursts, offering new criteria to differentiate between them in observations.

Findings

Extreme starbursts can mimic AGN mid-IR colors in two-band cuts.

Three-band cuts require very high ionization or gas density starbursts.

Proposed color cut effectively excludes extreme starbursts from AGN candidates.

Abstract

We conduct for the first time a theoretical investigation of the mid-infrared spectral energy distribution (SED) produced by dust heated by an active galactic nucleus (AGN) and an extreme starburst. These models employ an integrated modeling approach using photoionization and stellar population synthesis models in which both the line and emergent continuum is predicted from gas exposed to the ionizing radiation from a young starburst and an AGN. In this work, we focus on the infrared colors from the {\it Wide-field Infrared Survey Explorer}, predicting the dependence of the colors on the input radiation field, the ISM conditions, the obscuring column, and the metallicity. We find that an extreme starburst can mimic an AGN in two band mid-infrared color cuts employed in the literature. However, the three band color cuts employed in the literature require starbursts with extremely high ionization parameters or gas densities . We show that the extreme mid-IR colors seen in some blue compact dwarf galaxies are not due to metallicity but rather a combination of high ionization parameters and high column densities. Based on our theoretical calculations, we present a theoretical mid-infrared color cut that will exclude even the most extreme starburst that we have modeled in this work. The theoretical AGN demarcation region presented here can be used to identify elusive AGN candidates for future follow-up studies with the {\it James Webb Space Telescope (JWST)}. The full suite of simulated SEDs are available online.