Abstract

We present mid-infrared observations of active galactic nuclei (AGNs) in the GOODS fields, performed with the ∼Spitzer Space Telescope. These are the deepest infrared and X-ray fields to date and cover a total area of ∼0.1 deg2. ∼AGNs are selected on the basis of their hard (2-8 keV) X-ray emission. The median AGN infrared luminosity is at ∼least 10 times larger than the median for normal galaxies with the same redshift distribution, suggesting that the ∼infrared emission is dominated by the central nucleus. The X-ray-to-infrared luminosity ratios of GOODS AGNs, ∼most of which are at 0.5 ≤ z ≤ 1.5, are similar to the values obtained for AGNs in the local universe. The observed ∼infrared flux distribution has an integral slope of ∼ 1.5, and there are 1000 sources per square degree brighter than ∼50 μJy at ∼3-6 μm. The counts approximately match the predictions of models based on AGN unification, in ∼which the majority of AGNs are obscured. This agreement confirms that the faintest X-ray sources, which are ∼dominated by the host galaxy light in the optical, are obscured AGNs. Using these Spitzer data, the AGN contribution ∼to the extragalactic infrared background light is calculated by correlating the X-ray and infrared catalogs. This is ∼likely to be a lower limit given that the most obscured AGNs are missed in X-rays. We estimate the contribution of ∼AGNs missed in X-rays, using a population synthesis model, to be ∼45% of the observed AGN contribution, making ∼the AGN contribution to the infrared background at most ∼2%-10% in the 3-24 μm range, depending on wavelength, lower than most previous estimates. The AGN contribution to the infrared background remains roughly ∼constant with source flux in the IRAC bands but decreases with decreasing flux in the MIPS 24 μm band, where the ∼galaxy population becomes more important. © 2006. The American Astronomical Society. All rights reserved.