Spitzer IRS Spectra of Optically Faint Infrared Sources with Weak Spectral Features

Creators: Weedman, D. W.; Soifer, B. T.; Hao, Lei; Higdon, J. L.; Higdon, S. J. U.; Houck, J. R.; Le Floc'h, E.; Brown, M. J. I.; Dey, A.; Jannuzi, B. T.; Rieke, M.; Desai, V.; Bian, C.; Thompson, D.; Armus, L.; Teplitz, H.; Eisenhardt, P.; Willner, S. P.

Abstract

Spectra have been obtained with the low-resolution modules of the Infrared Spectrograph (IRS) on the Spitzer Space Telescope (Spitzer) for 58 sources having f_ν(24 μm) > 0.75 mJy. Sources were chosen from a survey of 8.2 deg^2 within the NOAO Deep Wide-Field Survey region in Boötes (NDWFS) using the Multiband Imaging Photometer (MIPS) on Spitzer. Most sources are optically very faint (I > 24 mag). Redshifts have previously been determined for 34 sources, based primarily on the presence of a deep 9.7 μm silicate absorption feature, with a median z of 2.2. Spectra are presented for the remaining 24 sources for which we were previously unable to determine a confident redshift because the IRS spectra show no strong features. Optical photometry from the NDWFS and infrared photometry with MIPS and the Infrared Array Camera on Spitzer (IRAC) are given, with K photometry from the Keck I telescope for some objects. The sources without strong spectral features have overall spectral energy distributions (SEDs) and distributions among optical and infrared fluxes that are similar to those for the sources with strong absorption features. Nine of the 24 sources are found to have feasible redshift determinations based on fits of a weak silicate absorption feature. Results confirm that the "1 mJy" population of 24 μm Spitzer sources that are optically faint is dominated by dusty sources with spectroscopic indicators of an obscured AGN rather than a starburst. There remain 14 of the 58 sources observed in Boötes for which no redshift could be estimated, and 5 of these sources are invisible at all optical wavelengths.

Additional Information

© 2006 The American Astronomical Society. Received 2006 May 16; accepted 2006 June 28. We thank D. Devost, G. Sloan, and P. Hall for help in improving our IRS spectral analysis. We thank the staff of the Keck Observatory for their assistance with obtaining the 2.2 µm observations. The W. M. Keck Observatory is operated as a scientific partnership between the California Institute of Technology, the University of California and NASA. It was made possible by the generous financial support of the W. M. Keck Foundation. We extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are priveleged to be guests. Without their generous hospitality, none of the Keck observations presented herein would have been possible. We thank NOAO for supporting the NOAO Deep Wide-Field Survey; A. D. and B. J. acknowledge support from NOAO, 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 primarily on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work by the IRS GTO team at Cornell University was provided by NASA through Contract Number 1257184 issued by JPL/Caltech.

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