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Published January 1, 2010 | Published
Journal Article Open

A Comparison of X-ray and Mid-Infrared Selection of Obscured Active Galactic Nuclei


We compare the relative merits of active galactic nuclei (AGNs) selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated photometric and spectroscopic optical follow-up are drawn from a subset of fields studied as part of the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program. Mid-infrared data in these fields are derived from targeted and archival Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the Infrared Array Camera (IRAC) color–color AGN "wedge" selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGNs by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGNs in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGNs are detected at X-ray energies in the moderately deep ({t_(exp} ≈ 100 ks) SEXSI Chandra data. We hypothesize that IRAC sources with AGN colors that lack X-ray detections are predominantly high-luminosity AGNs that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2–8 keV band 3 times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. The hardness ratio of the wedge-selected stack is consistent with moderate intrinsic obscuration, but is not suggestive of a highly obscured, Compton-thick source population. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray, and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR.

Additional Information

© 2010 American Astronomical Society. Received 2009 March 15; accepted 2009 November 12; published 2009 December 14. This work is based on observations made with Spitzer, which is operated by the Jet Propulsion Laboratory, California Institute of the Technology under contract with the National Aeronautics and Space Administration (NASA). Support for this work was provided by NASA through award number 1314516 issued by JPL/Caltech. The authors thank the anonymous referee for his/her careful read and unusually diligent comments as well as Lewis Kotredes for assistance with Spitzer data reduction. M.E.E. acknowledges support from the NASA Postdoctoral Program. Facilities:: Spitzer, CXO, Keck:I, and Keck:II.

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