The Active Galactic Nuclei Contribution to the Mid-Infrared Emission of Luminous Infrared Galaxies

June 2006 • 2006ApJ...644..143B

Authors • Brand, K. • Dey, A. • Weedman, D. • Desai, V. • Le Floc'h, E. • Jannuzi, B. T. • Soifer, B. T. • Brown, M. J. I. • Eisenhardt, P. • Gorjian, V. • Papovich, C. • Smith, H. A. • Willner, S. P. • Cool, R. J.

Abstract • 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.

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Vandana Desai

Senior Scientist