Modeling Mid-Infrared Diagnostics of Obscured Quasars and Starbursts

TL;DR

This study uses dust radiative transfer models of galaxy mergers to evaluate mid-infrared diagnostics for distinguishing active galactic nuclei from starbursts, highlighting the potential of combined spectral features to estimate AGN power.

Contribution

It introduces a new diagnostic approach combining multiple mid-infrared features to better estimate AGN contribution in obscured ULIRGs, based on detailed simulations.

Findings

Mid-infrared diagnostics are affected by dust obscuration and viewing angle.

A combination of the 9.7 micron silicate feature, PAH strength, and near-IR slope can constrain AGN fraction.

The proposed diagnostic can estimate AGN power as accurately as hard X-ray flux.

Abstract

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