The active galactic nuclei contribution to the mid-infrared emission of luminous infrared galaxies

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 mu m sources are selected from a Spitzer MIPS survey of the NOAO Deep Wide-Field Survey Bootes field and have corresponding 8 mu m data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 to 8 mu m flux ratio. The X-ray-detected sources fall within the peak corresponding to a flat spectrum in nu f(nu), implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 to 8 mu m flux ratio is likely due to LIRGs whose IR emission is powered by starbursts. The 24 mu m emission is increasingly dominated by AGN at higher 24 mu m flux densities (f(24)): the AGN fraction of the z > 0.6 sources increases from 9% at f(24) approximate to 0.35 mJy to 74% +/- 20% at f(24) approximate to 3 mJy, in good agreement with model predictions. Deep 24 mu m, small-area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for similar to 3%-7% of the total 24 mu m background.