The origin of the 24 μm excess in red galaxies

Kate Brand, John Moustakas, Lee Armus, Roberto J. Assef, Michael J.I. Brown, Richard R. Cool, Vandana Desai, Arjun Dey, Emeric Le Floc'H, Buell T. Jannuzi, Christopher S. Kochanek, Jason Melbourne, Casey J. Papovich, B. T. Soifer

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)340-346
Number of pages7
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2009


  • galaxies: elliptical and lenticular, cD
  • galaxies: starburst
  • infrared: galaxies
  • quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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