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Published June 10, 2008 | Published
Journal Article Open

Spitzer mid-infrared spectroscopy of distant X-ray luminous active galactic nuclei


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 surveys in the NOAO Deep Wide-Field Survey Boötes field and were selected from their bright X-ray fluxes to host luminous AGNs. None of the spectra show significant emission from polycyclic aromatic hydrocarbons (PAHs; 6.2 μm equivalent widths <0.2 μm), consistent with their infrared emission being dominated by AGNs. Nine of the X-ray sources show 9.7 μm silicate absorption features. Their redshifts are in the range 0.9 < z < 2.6, implying infrared luminosities of log LIR = 12.5–13.6 L⊙. The average silicate absorption 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-infrared 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 the column density of X-ray-absorbing gas.

Additional Information

© 2008 The American Astronomical Society. Received 2007 November 12; accepted 2008 January 29. We thank our colleagues on the NDWFS, XBootes, MIPS, IRS, AGES, and IRAC teams. K. B. is supported by the Giacconi Fellowship at STScI. This research is supported by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. Support for this work by the IRS GTO team at Cornell University was provided by NASA through contract No. 1257184 issued by JPL/Caltech. 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 survey of the Bootes region was obtained using GTO time provided by the Spitzer Infrared Spectrograph Team (PI: J. Houck) and M. Rieke. 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. We thank the anonymous referee for useful comments.

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