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The Origin of the 24-micron 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
; | | Date: |
10 Nov 2008 | | Abstract: | Observations with the Spitzer Space Telescope have revealed a population of
red-sequence galaxies with a significant excess in their 24-micron 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 24-micron infrared excess (f24>0.3 mJy). 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 Spitzer Shallow Survey, we find that 64%
of the 24-micron excess red galaxies are likely to have strong PAH emission
features in the 8-micron IRAC band. This fraction is significantly larger than
the 5% of red galaxies with f24<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-micron 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 24-micron 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-micron excess red galaxies are edge-on
spiral galaxies with high optical extinctions. The remaining sources are likely
to be red galaxies whose 24-micron emission comes from a combination of
obscured AGN and star-formation activity. | | Source: | arXiv, 0811.1398 | | Services: | Forum | Review | PDF | Favorites | |
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