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 0.1
5<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 IRAC 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.
We present mid-infrared spectroscopy of a sample of 16 optically faint infrared luminous galaxies obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. These sources were jointly selected from Spitzer and Chandra imaging surve
ys in the NDWFS Bootes field and were selected from their bright X-ray fluxes to host luminous AGN. None of the spectra show significant emission from polycyclic aromatic hydrocarbons (PAHs; 6.2um equivalent widths <0.2um), consistent with their infrared emission being dominated by AGN. Nine of the X-ray sources show 9.7um silicate absorption features. Their redshifts are in the range 0.9<z<2.6, implying infrared luminosities of log(L{IR})=12.5-13.6 solar luminosities. The average silicate absorption strength is not as strong as that of previously targeted optically faint infrared luminous galaxies with similar mid-infrared luminosities implying that the X-ray selection favors sources behind a smaller column of Si-rich dust than non-X-ray selection. Seven of the X-ray sources have featureless power-law mid-IR spectra. We argue that the featureless spectra likely result from the sources having weak or absent silicate and PAH features rather than the sources lying at higher redshifts where these features are shifted out of the IRS spectral window. We investigate whether there are any correlations between X-ray and infrared properties and find that sources with silicate absorption features tend to have fainter X-ray fluxes and harder X-ray spectra, indicating a weak relation between the amount of silicate absorption and column density of X-ray-absorbing gas.
We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint, infrared luminous galaxies selected from a Spitzer MIPS 70um imaging survey of the NDWFS Bootes field. These are the first Spitzer IRS s
pectra presented of distant 70um-selected sources. All the galaxies lie at redshifts 0.3<z<1.3 and have very large infrared luminosities of L_IR~ 0.1-17 x 10^12 solar luminosities. Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons (PAHs). The average IRS spectrum of these sources is characteristic of classical starburst galaxies, but with much larger infrared luminosities. The PAH luminosities of L(7.7) ~ 0.4 - 7 x 10^11 solar luminosities imply star formation rates of ~ 40 - 720 solar masses per year. Four of the galaxies show deep 9.7um silicate absorption features and no significant PAH emission features (6.2um equivalent widths < 0.03um). The large infrared luminosities and low f70/f24 flux density ratios suggests that these sources have AGN as the dominant origin of their large mid-infrared luminosities, although deeply embedded but luminous starbursts cannot be ruled out. If the absorbed sources are AGN-dominated, a significant fraction of all far-infrared bright, optically faint sources may be dominated by AGN.
