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Modeling Mid-infrared Diagnostics of Obscured Quasars and Starbursts

Snyder, Gregory F. and Hayward, Christopher C. and Sajina, Anna and Jonsson, Patrik and Cox, Thomas J. and Hernquist, Lars and Hopkins, Philip F. and Yan, Lin (2013) Modeling Mid-infrared Diagnostics of Obscured Quasars and Starbursts. Astrophysical Journal, 768 (2). Art. No. 168. ISSN 0004-637X.

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We analyze the link between active galactic nuclei (AGNs) 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 ~ 2-4. This highly obscured burst begins star-formation-dominated with significant polycyclic aromatic hydrocarbon (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 PAHs, and enhances the 9.7 μm 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 μm, 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Hayward, Christopher C.0000-0003-4073-3236
Sajina, Anna0000-0002-1917-1200
Hernquist, Lars0000-0001-6950-1629
Hopkins, Philip F.0000-0003-3729-1684
Yan, Lin0000-0003-1710-9339
Additional Information:© 2013 American Astronomical Society. Received 2012 October 7; accepted 2013 April 5; published 2013 April 25. This research has made use of NASA’s Astrophysics Data System. The computations in this paper were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. We thank the anonymous referee for valuable suggestions that improved this paper.
Group:Infrared Processing and Analysis Center (IPAC), TAPIR
Funding AgencyGrant Number
Harvard UniversityUNSPECIFIED
Subject Keywords:dust, extinction; galaxies: interactions; galaxies: starburst; infrared: galaxies; quasars: general; radiative transfer
Issue or Number:2
Record Number:CaltechAUTHORS:20130605-082348981
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Official Citation:Modeling Mid-infrared Diagnostics of Obscured Quasars and Starbursts Gregory F. Snyder, Christopher C. Hayward, Anna Sajina, Patrik Jonsson, Thomas J. Cox, Lars Hernquist, Philip F. Hopkins, and Lin Yan doi:10.1088/0004-637X/768/2/168
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38799
Deposited By: Ruth Sustaita
Deposited On:05 Jun 2013 15:35
Last Modified:04 Nov 2019 01:50

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