Mid‐Infrared Properties of X‐Ray Sources in the Extended Groth Strip

Mid-infrared observations of Active Galactic Nuclei (AGN) are important forunderstanding of the physical conditions around the central accretion engines.Chandra and XMM-Newton X-ray observations of a 300 arcmin^2 region in theExtended Groth Strip are used to select a sample of~150 AGN. The Spitzerinstruments IRAC and MIPS detect 68-80% of these sources, which show a widerange of mid-infrared properties. About 40% of the sources have red power-lawspectral energy distributions (f_nu ~ nu^alpha, alpha<0) in the 3.6-8 um IRACbands. In these sources the central engine dominates the emission at both X-rayand IR wavelengths. Another 40% of the sources have blue mid-IR spectral energydistributions (alpha>0) with their infrared emission dominated by the hostgalaxy; the remaining 20% are not well-fit by a power law. Published IRAC colorcriteria for AGN select most of the red sources, but only some of the bluesources. As with all other known methods, selecting AGN with mid-IR colors willnot produce a sample that is simultaneously complete and reliable. The IRAC SEDtype does not directly correspond to X-ray spectral type (hard/soft). Themid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, andoptically-faint sources vary widely and, for the most part, are not distinctfrom those of the general X-ray/infrared source population. X-ray sources emit6-11% of the integrated mid--IR light, making them significant contributors tothe cosmic infrared background.Comment: Replaced to correct minor error in Figure 3 axis label and typo in author list. ApJ, in press; 40 pages, 12 figure