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Published March 1, 2009 | Published
Journal Article Open

The origin of the 24 μm excess in red galaxies


Observations with the Spitzer Space Telescope have revealed a population of red sequence galaxies with a significant excess in their 24 μm 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 24 μm infrared excess (f_(24) ≥ 0.3 mJy). We determine the prevalence of active galactic nucleus (AGN) and star-formation activity in all the AGES galaxies using optical line diagnostics and mid-IR color–color criteria. Using the IRAC color–color diagram from the Spitzer Shallow Survey, we find that 64% of the 24 μm excess red galaxies are likely to have strong polycyclic aromatic hydrocarbon (PAH) emission features in the 8 μm IRAC band. This fraction is significantly larger than the 5% of red galaxies with f_(24) < 0.3 mJy that are estimated to have strong PAH emission, suggesting that the infrared emission is largely due to star-formation processes. Only 15% of the 24 μm excess red galaxies have optical line diagnostics characteristic of star formation (64% are classified as AGN and 21% are unclassifiable). The difference between the optical and infrared results suggests that both AGN and star-formation activity are occurring simultaneously in many of the 24 μm 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 24 μm excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24 μm emission comes from a combination of obscured AGN and star-formation activity.

Additional Information

© 2009. The American Astronomical Society. Received 2008 August 22; accepted 2008 November 7; published 2009 March 2. We thank our colleagues on the NDWFS, MIPS, AGES, and IRAC teams. K.B. is supported by the Giacconi fellowship at STScI. J.M. acknowledges funding support from NASA-06- GALEX06-0030 and Spitzer G05-AR-50443. This research is partially supported by the National Optical Astronomy Observatory which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under a cooperative agreement with the National Science Foundation. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The Spitzer/MIPS survey of the Boötes region was obtained using GTO time provided by the Spitzer Infrared Spectrograph Team (P.I.: James Houck) and by M. Rieke. AGES is a collaboration between scientists at the Harvard-Smithsonian Center for Astrophysics, Steward Observatory, the National Optical Astronomical Observatory, the Ohio State University, and the Jet Propulsion Laboratory. We thank the anonymous referee for his/her useful comments.

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