THE ORIGIN OF THE 24 μm EXCESS IN RED GALAXIES

Open Access
Publisher Website
Google Scholar
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 Bootes 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 color- color 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 star- formation (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. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: starburst — infrared: galaxies — quasars: general