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.
Highlights
Understanding the link between supermassive black holes (SMBHs) and their host galaxies is essential for deciphering the formation and evolution of galaxies
A key difference between Hopkins et al (2005) and the present work is that their assumptions correspond to the “alternate interstellar medium (ISM)” treatment we described in Section 2.3, where the cold clumps in the ISM, and often a significant fraction of the dust mass, are assumed to have a small volume filling factor
We used the detailed information contained in high-resolution hydrodynamical merger simulations to analyze the impact of galaxy-scale evolution of dust on interpretations of AGN power in ultra-luminous infrared galaxies (ULIRGs) derived from the mid-infrared
Summary
Understanding the link between supermassive black holes (SMBHs) and their host galaxies is essential for deciphering the formation and evolution of galaxies. A key prediction from this framework is that a galaxy experiencing rapid inflow of cold gas can evolve through various classes of starbursts and active galactic nuclei (AGNs), such as ultra-luminous infrared galaxies (ULIRGs) and quasars (QSOs), and that these phases are connected in an evolutionary sequence (e.g., Sanders et al 1988). With such a model, both a starburst and a heavily obscured AGN can co-exist (Hopkins et al 2006). The most extreme starbursts are the ULIRGs (LIR > 1012 L ; Sanders et al 1988), which are almost exclusively the
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