SPITZER'S CONTRIBUTION TO THE AGN POPULATION

Infrared selection is a potentially powerful way to identify heavily obscured AGNs missed in even the deepest X-ray surveys. Using a 24 mu m-selected sample in GOODS-S, we test the reliability and completeness of three infrared AGN selection methods: (1) IRAC color-color selection, (2) IRAC power-law selection, and (3) IR-excess selection; we also evaluate a number of IR-excess approaches. We find that the vast majority of non-power-law IRAC color-selected AGN candidates in GOODS-S have colors consistent with those of star-forming galaxies. Contamination by star-forming galaxies is most prevalent at low 24 mu m flux densities (similar to 100 mu Jy) and high redshifts (z similar to 2), but the fraction of potential contaminants is still high (similar to 50%) at 500 mu Jy, the highest flux density probed reliably by our survey. AGN candidates selected via a simple, physically motivated power-law criterion ("power-law galaxies," or PLGs), however, appear to be reliable. We confirm that the IR-excess methods successfully identify a number of AGNs, but we also find that such samples may be significantly contaminated by star-forming galaxies. Adding only the secure Spitzer-selected PLG, color-selected, IR-excess, and radio/IR-selected AGN candidates to the deepest X-ray-selected AGN samples directly increases the number of known X-ray AGNs (84) by 54%-77%, and implies an increase to the number of 24 mu m-detected AGNs of 71%-94%. Finally, we show that the fraction of MIR sources dominated by an AGN decreases with decreasing MIR flux density, but only down to f(24 mu m) = 300 mu Jy. Below this limit, the AGN fraction levels out, indicating that a nonnegligible fraction (similar to 10%) of faint 24 mu m sources (the majority of which are missed in the X-ray) are powered not by star formation, but by the central engine. The fraction of all AGNs ( regardless of their MIR properties) exceeds 15% at all 24 mu m flux densities.