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 mu m emission compared to what is expected from an old stellar population. We identify similar to 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 Bootes field. Using Spitzer MIPS, we classify 89 (similar to 10%) with 24 mu m 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 mu m excess red galaxies are likely to have strong polycyclic aromatic hydrocarbon (PAH) emission features in the 8 mu m 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 mu m 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 mu m 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 similar to 40% of the 24 mu m excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24 mu m emission comes from a combination of obscured AGN and star-formation activity.