2006ApJ...642..126B

Mid-infrared observations of active galactic nuclei (AGNs) are important for understanding the physical conditions around the central accretion engines. Chandra and XMM-Newton X-ray observations of a 300 arcmin² region in the extended Groth strip are used to select a sample of ∼150 AGNs. The Spitzer instruments IRAC and MIPS detect 68%-80% of these sources, which show a wide range of mid-infrared properties. About 40% of the sources have red power-law spectral energy distributions (f_ν∝ν^α, α<0) in the 3.6-8 µm IRAC bands. In these sources the central engine dominates the emission at both X-ray and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy distributions (α>0) with their infrared emission dominated by the host galaxy; the remaining 20% are not well fit by a power law. Published IRAC color criteria for AGNs select most of the red sources, but only some of the blue sources. As with all other known methods, selecting AGNs with mid-IR colors will not produce a sample that is simultaneously complete and reliable. The IRAC SED type does not directly correspond to X-ray spectral type (hard/soft). The mid-IR properties of X-ray-detected Lyman break, radio, submillimeter, and optically faint sources vary widely, and for the most part are not distinct from those of the general X-ray/infrared source population. X-ray sources emit 6%-11% of the integrated mid-IR light, making them significant contributors to the cosmic infrared background.