The Agn Contribution to the Mid-ir 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 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 μm to 8 μm flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in νfν , implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 μm to 8 μm flux ratio is likely due to LIRGs whose infrared 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. Subject headings: galaxies: active — galaxies: starburst — X-rays — infrared: galaxies