Mid-Infrared Selection of Active Galaxies


Stern D, Eisenhardt P, Gorjian V, Kochanek CS, Caldwell N, Eisenstein D, Brodwin M, Brown MJI, Cool R, Dey A, et al. Mid-Infrared Selection of Active Galaxies. The Astrophysical Journal. 2005;631 :163-168.

Date Published:

September 1, 200


Mid-infrared photometry provides a robust technique for identifyingactive galaxies. While the ultraviolet to mid-infrared (λ<~5μm) continuum of stellar populations is dominated by the compositeblackbody curve and peaks at approximately 1.6 μm, the ultraviolet tomid-infrared continuum of active galactic nuclei (AGNs) is dominated bya power law. Consequently, with a sufficient wavelength baseline, onecan easily distinguish AGNs from stellar populations. Mirroring thetendency of AGNs to be bluer than galaxies in the ultraviolet, wheregalaxies (and stars) sample the blue, rising portion of stellar spectra,AGNs tend to be redder than galaxies in the mid-infrared, where galaxiessample the red, falling portion of the stellar spectra. We report onSpitzer Space Telescope mid-infrared colors, derived from the IRACShallow Survey, of nearly 10,000 spectroscopically identified sourcesfrom the AGN and Galaxy Evolution Survey. On the basis of thisspectroscopic sample, we find that simple mid-infrared color criteriaprovide remarkably robust separation of active galaxies from normal