Mid-IR colors of AGNs in the MUSYC ECDF-S

Using a statistical matching technique, the likelihood ratio, over 900 IRAC counterparts were identified with a new MUSYC X-ray source catalog that includes ~1000 published X-ray sources in the Chandra Deep Field-South and Extended Chandra Deep Field-South. Most X-ray-selected AGNs have IRAC spectral shapes consistent with power-law slopes, f_{nu}_{prop.to}{nu}^{alpha}^, and display a wide range of colors, -2<=alpha}<=2 Although X-ray sources typically fit to redder (more negative {alpha}) power laws than non-X-ray-detected galaxies, more than 50% do have flat or blue (galaxy-like) spectral shapes in the observed 3-8um band.

Only a quarter of the X-ray-selected AGNs detected at 24um are well fit by featureless red power laws in the observed 3.6-24um, likely the subset of our sample whose infrared spectra are dominated by emission from the central AGN region. Most IRAC color selection criteria fail to identify the majority of X-ray-selected AGNs, finding only the more luminous AGNs, the majority of which have broad emission lines. In deep surveys, these color selection criteria select 10%-20% of the entire galaxy population and miss many moderate-luminosity AGNs.