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ApJ, in press Preprint typeset using L ATEX style emulateapj v. 10/09/06 THE ORIGIN OF THE 24µM EXCESS IN RED GALAXIES

By Kate Br, John Moustakas, Lee Armus, Roberto J. Assef, Michael J. I. Brown, Richard R. Cool, Ana Desai, Arjun Dey, Emeric Le Floc’h, Buell T. Jannuzi, Christopher S. Kochanek and B. T. Soifer

Abstract

Observations with the Spitzer Space Telescope have revealed a population of red sequence galaxies with a significant excess in their 24µm emission compared to what is expected from an old stellar population. We identify ∼900 red galaxies with 0.15 ≤ z ≤ 0.3 from the AGN and Galaxy Evolution Survey (AGES) selected from the NOAO Deep Wide-Field Survey Boötes field. Using Spitzer MIPS, we classify 89 (∼10%) with 24µm infrared excess (f24 ≥0.3mJy). We determine the prevalence of AGN and star-formation activity in all the AGES galaxies using optical line diagnostics and mid-IR colorcolor criteria. Using the IRAC color-color diagram from the IRAC Shallow Survey, we find that 64 % of the 24µm excess red galaxies are likely to have strong PAH emission features in the 8µm IRAC band. This fraction is significantly larger than the 5 % of red galaxies with f24 <0.3mJy that are estimated to have strong PAH emission, suggesting that the infrared emission is largely due to star-formation processes. Only 15 % of the 24µm excess red galaxies have optical line diagnostics characteristic of starformation (64 % are classified as AGN and 21 % are unclassifiable). The difference between the optical and infrared results suggest that both AGN and star-formation activity is occurring simultaneously in many of the 24µm excess red galaxies. These results should serve as a warning to studies that exclusively use optical line diagnostics to determine the dominant emission mechanism in the infrared and other bands. We find that ∼40 % of the 24µm excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24µm emission comes from a combination of obscured AGN and star-formation activity

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