Modeling Mid-Infrared Diagnostics of Obscured Quasars and Starbursts. (arXiv:1210.6347v1 [astro-ph.CO])

Modeling Mid-Infrared Diagnostics of Obscured Quasars and Starbursts. (arXiv:1210.6347v1 [astro-ph.CO]):
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 effect 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 meant to bracket the properties of ULIRGs. Our more obscured
starburst begins SF-dominated with significant PAH emission, and ends with a
~10^9 year 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. Some identify
relatively unobscured AGN where the direct torus emission is observed, while
others indicate more highly obscured AGN. We show that a combination of the
extinction feature at 9.7 microns, 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 procedure, 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.