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Published January 20, 2008 | Published
Journal Article Open

Spitzer mid-infrared spectroscopy of 70 μm-selected distant luminous infrared galaxies


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 70 μm imaging survey of the NDWFS Boötes field. These are the first Spitzer IRS spectra presented of distant 70 μm-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) × 10^(12)Lסּ. 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 ν νL_ν(7.7 μ m) ~ (0.4–7) × 10^(11) Lסּ imply star formation rates of ~40-720 Mסּ yr^(−1). Four of the galaxies show deep 9.7 μm silicate absorption features and no significant PAH emission features (6.2 μm equivalent widths <0.03 μm). The large infrared luminosities and low ν νf_ν(70 μ m)/νf_ν(24 μ m) flux density ratios suggests that these sources have AGNs 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 AGNs.

Additional Information

© 2008. The American Astronomical Society. Received 2007 August 6; accepted 2007 September 17. We thank our colleagues on the NDWFS, MIPS, IRS, AGES, and IRAC teams. K. B. is supported by the Giacconi fellowship at STScI. Support for this work by the IRS GTO team at Cornell University was provided by NASA through contract 1257184 issued by JPL/Caltech. This research is partially supported by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The Spitzer MIPS and IRAC surveys of the Boo¨tes region were obtained using GTO time provided by the Spitzer Infrared Spectrograph Team ( PI: James Houck), M. Rieke, and the IRAC Team ( PI: G. Fazio). IRAC is supported in part through contract 960541 issued by JPL. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.

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