Spitzer's contribution to the AGN population



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Donley, J. L. and Rieke, G. H. and Pérez González, Pablo Guillermo and Barro, Guillermo (2008) Spitzer's contribution to the AGN population. Astrophysical journal, 687 (1). pp. 111-132. ISSN 0004-637X

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Official URL: http://dx.doi.org/10.1086/591510


Infrared selection is a potentially powerful way to identify heavily obscured AGNs missed in even the deepest X-ray surveys. Using a 24 μm-selected sample in GOODS-S, we test the reliability and completeness of three infrared AGN selection methods: (1) IRAC color-color selection, (2) IRAC power-law selection, and (3) IR-excess selection; we also evaluate a number of IR-excess approaches. We find that the vast majority of non-power-law IRAC color-selected AGN candidates in GOODS-S have colors consistent with those of star-forming galaxies. Contamination by star-forming galaxies is most prevalent at low 24 μm flux densities (~100 μJy) and high redshifts (z ~ 2), but the fraction of potential contaminants is still high (~50%) at 500 μJy, the highest flux density probed reliably by our survey. AGN candidates selected via a simple, physically motivated power-law criterion ("power-law galaxies," or PLGs), however, appear to be reliable. We confirm that the IR-excess methods successfully identify a number of AGNs, but we also find that such samples may be significantly contaminated by star-forming galaxies. Adding only the secure Spitzer-selected PLG, color-selected, IR-excess, and radio/IR-selected AGN candidates to the deepest X-ray-selected AGN samples directly increases the number of known X-ray AGNs (84) by 54%-77%, and implies an increase to the number of 24 μm-detected AGNs of 71%-94%. Finally, we show that the fraction of MIR sources dominated by an AGN decreases with decreasing MIR flux density, but only down to f_24 μ m = 300 μJy. Below this limit, the AGN fraction levels out, indicating that a nonnegligible fraction (~10%) of faint 24 μm sources (the majority of which are missed in the X-ray) are powered not by star formation, but by the central engine. The fraction of all AGNs (regardless of their MIR properties) exceeds 15% at all 24 μm flux densities.

Item Type:Article
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© 2008 The American Astronomical Society. All rights reserved.
We thank M. Polletta for providing templates, M. Dickinson, and D. Alexander for providing the list of Daddi et al. (2007a, 2007b) sources. We also thank D. Alexander, F. Fiore, M. Lacy, D. Stern, and the anonymous referee for discussions and comments that improved the paper. Finally, we thank Caltech/JPL for support through contract 1255094 to the University of Arizona. P. G. P.-G. acknowledges support from the Spanish Programa Nacional de Astronomía y Astrofísica under grant AYA 2006Y02358 and AYA 2006Y15698YC02Y02, and from the Ramón y Cajal Program financed by the Spanish Government and the European Union.

Uncontrolled Keywords:Active galactic nuclei; Deep field-south; Ultraluminous infrared galaxies; Star-forming galaxies; Spectral energy-distributions; Supermassive black-holes; Small magellanic cloud; Extremely red objects; 2-10 kev luminosity; Digital sky survey
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:38668
Deposited On:01 Aug 2016 15:10
Last Modified:10 Dec 2018 15:05

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