The Active Galactic Nuclei Contribution to the Mid-Infrared Emission of Luminous Infrared Galaxies
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We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z > 0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 m sources are selected from a Spitzer MIPS survey of the NOAO Deep Wide-Field Survey Botes field and have corresponding 8 m data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 to 8 m flux ratio. The X-ray-detected sources fall within the peak corresponding to a flat spectrum in vf v, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 to 8 m flux ratio is likely due to LIRGs whose IR emission is powered by starbursts. The 24 m emission is increasingly dominated by AGN at higher 24 m flux densities (f 24): the AGN fraction of the z > 0.6 sources increases from 9% at f24 0.35 mJy to 74% 20% at f24 3 mJy, in good agreement with model predictions. Deep 24 m, small-area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for 3%-7% of the total 24 m background. 2006. The American Astronomical Society. All rights reserved.