The Quest for Obscured AGN at Cosmological Distances: Infrared Power-Law Galaxies

We summarize multiwavelength properties of a sample of galaxies in the CDF-N and CDF-S whose spectral energy distributions (SEDs) exhibit the characteristic power-law behavior expected for AGN in the Spitzer/IRAC 3.6?8 μm bands. AGN selected this way tend to comprise the majority of high x-ray luminosity AGN, whereas AGN selected via other IRAC color–color criteria might contain more starformation dominated galaxies. Approximately half of these IR power-law galaxies in the CDF-S are detected in deep (1 Ms) Chandra x-ray imaging, although in the CDF-N (2 Ms) about 77% are detected at the 3 σlevel. The SEDs and x-ray upper limits of the sources not detected in x-rays are consistent with those of obscured AGN, and are significantly different from those of massive star-forming galaxies. About 40% of IR power-law galaxies detected in x-rays have SEDs resembling that of an optical QSO and morphologies dominated by bright point source emission. The remaining 60% have SEDs whose UV and optical continuum are much steeper (obscured) and more extended morphologies than those detected in x-rays. Most of the IR power-law galaxies not detected in x-rays have IR (8?1000 μm) above 1012 L_, and x-ray (upper limits) to mid-IR ratios similar to those of local warm (i.e. hosting an AGN) ULIRGs. The SED shapes of power-law galaxies are consistent with the obscured fraction (4:1) as derived from the x-ray column densities, if we assume that all the sources not detected in x-rays are heavily absorbed. IR power-law galaxies may account for between 20% and 50% of the predicted number density of mid-IR detected obscured AGN. The remaining obscured AGN probably have rest-frame SEDs dominated by stellar emission.