Infrared selection is a potentially powerful way to identify heavily obscured AGN missed in even the deepest X-ray surveys .
Using a 24 µm-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 infrared 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 µm flux densities ( \sim 100 \mu Jy ) and high redshifts ( z \sim 2 ) , but the fraction of potential contaminants is still high ( \sim 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 ( PLGs ) , however , appear to be reliable .
We confirm that the infrared excess methods successfully identify a number of AGN , 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 AGN ( 84 ) by 54 - 77 \% , and implies an increase to the number of 24 µm-detected AGN 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 ~ { } \micron } = 300 \mu Jy .
Below this limit , the AGN fraction levels out , indicating that a non-negligible fraction ( \sim 10 \% ) of faint 24 µm 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 AGN ( regardless of their MIR properties ) exceeds 15 % at all 24 µm flux densities .