  Mid-infrared photometry provides a robust technique for identifying active
galaxies. While the ultraviolet to mid-infrared continuum of normal galaxies is
dominated by the composite stellar black body curve and peaks at approximately
1.6 microns, the ultraviolet to mid-infrared continuum of active galaxies is
dominated by a power law. Consequently, with sufficient wavelength baseline,
one can easily distinguish AGN from stellar populations. Mirroring the tendency
of AGN to be bluer than galaxies in the ultraviolet, where galaxies (and stars)
sample the blue, rising portion of stellar spectra, AGN tend to be redder than
galaxies in the mid-infrared, where galaxies sample the red, falling portion of
the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors,
derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically
identified sources from the AGN and Galaxy Evolution Survey. Based on this
spectroscopic sample, we find that simple mid-infrared color criteria provide
remarkably robust separation of active galaxies from normal galaxies and
Galactic stars, with over 80% completeness and less than 20% contamination.
Considering only broad-lined AGN, these mid-infrared color criteria identify
over 90% of spectroscopically identified quasars and Seyfert 1s. Applying these
color criteria to the full imaging data set, we discuss the implied surface
density of AGN and find evidence for a large population of optically obscured
active galaxies.
