The Origin of the 24 μm Excess in Red Galaxies

March 2009 • 2009ApJ...693..340B

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

Abstract • 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.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2009ApJ...693..340B
• PDF https://stacks.iop.org/0004-637X/693/340/pdf
• PREPRINT http://arxiv.org/abs/0811.1398
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2009ApJ...693..340B
• DATA https://irsa.ipac.caltech.edu/bibdata/2009/B/2009ApJ...693..340B.html
• ELECTR https://doi.org/10.1088%2F0004-637X%2F693%2F1%2F340
• SPIRES http://inspirehep.net/search?p=find+eprint+arXiv:0811.1398