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We analyze the link between active galactic nuclei (AGN) and mid-infraredflux using dust radiative transfer calculations of starbursts realized inhydrodynamical simulations. Focusing on the effects of galaxy dust, we evaluatediagnostics commonly used to disentangle AGN and star formation inultraluminous infrared galaxies (ULIRGs). We examine these quantities as afunction of time, viewing angle, dust model, AGN spectrum, and AGN strength inmerger simulations representing two possible extremes of the ULIRG population:one is a typical gas-rich merger at z ~ 0, and the other is characteristic ofextremely obscured starbursts at z ~ 2 to 4. This highly obscured burst beginsstar-formation-dominated with significant PAH emission, and ends with a ~10^9yr period of red near-IR colors. At coalescence, when the AGN is most luminous,dust obscures the near-infrared AGN signature, reduces the relative emissionfrom polycyclic aromatic hydrocarbons (PAHs), and enhances the 9.7 micronabsorption by silicate grains. Although generally consistent with previousinterpretations, our results imply none of these indicators can unambiguouslyestimate the AGN luminosity fraction in all cases. Motivated by thesimulations, we show that a combination of the extinction feature at 9.7micron, the PAH strength, and a near-infrared slope can simultaneouslyconstrain the AGN fraction and dust grain distribution for a wide range ofobscuration. We find that this indicator, accessible to the James Webb SpaceTelescope, may estimate the AGN power as tightly as the hard X-ray flux alone,thereby providing a valuable future cross-check and constraint for largesamples of distant ULIRGs.

Modeling Mid-Infrared Diagnostics of Obscured Quasars and Starbursts