2006ApJ...644..143B

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 µm sources are selected from a Spitzer MIPS survey of the NOAO Deep Wide-Field Survey Boötes field and have corresponding 8 µm data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 to 8 µm flux ratio. The X-ray-detected sources fall within the peak corresponding to a flat spectrum in νf_ν, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 to 8 µm flux ratio is likely due to LIRGs whose IR emission is powered by starbursts. The 24 µm emission is increasingly dominated by AGN at higher 24 µm flux densities (f₂₄): 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 µm, small-area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ∼3%-7% of the total 24 µm background.