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(abridged) Infrared selection is a potentially powerful way to identifyheavily obscured AGN missed in even the deepest X-ray surveys. Using a 24micron-selected sample in GOODS-S, we test the reliability and completeness ofthree infrared AGN selection methods: (1) IRAC color-color selection, (2) IRACpower-law selection, and (3) IR-excess selection; we also evaluate a number ofinfrared excess approaches. We find that the vast majority of non-power-lawIRAC color-selected AGN candidates in GOODS-S have colors consistent with thoseof star-forming galaxies. Contamination by star-forming galaxies is mostprevalent at low 24 micron flux densities (~100 uJy) and high redshifts (z~2),but the fraction of potential contaminants is still high (~50%) at 500 uJy, thehighest flux density probed reliably by our survey. AGN candidates selected viaa simple, physically-motivated power-law criterion (PLGs), however, appear tobe reliable. We confirm that the infrared excess methods successfully identifya number of AGN, but we also find that such samples may be significantlycontaminated by star-forming galaxies. Adding only the secure Spitzer-selectedPLG, color-selected, IR-excess, and radio/IR-selected AGN candidates to thedeepest X-ray-selected AGN samples directly increases the number of known X-rayAGN (84) by 54-77%, and implies an increase to the number of 24 micron-detectedAGN of 71-94%. Finally, we show that the fraction of MIR sources dominated byan AGN decreases with decreasing MIR flux density, but only down to f_24 = 300uJy. Below this limit, the AGN fraction levels out, indicating that anon-negligible fraction (~10%) of faint 24 micron sources (the majority ofwhich are missed in the X-ray) are powered not by star formation, but by thecentral engine.

Spitzer's Contribution to the AGN Population