Redshift distribution of extragalactic 24 μm sources

Creators: Desai, Vandana; Soifer, B. T.; Dey, Arjun; Jannuzi, Buell T.; Le Floc'h, Emeric; Bian, Chao; Brand, Kate; Brown, Michael J. I.; Armus, Lee; Weedman, Dan W.; Cool, Richard; Stern, Daniel; Brodwin, Mark

Abstract

We present the redshift distribution of a complete, unbiased sample of 24 μm sources down to f_ν(24 μm) = 300 μJy (5 σ). The sample consists of 591 sources detected in the Boötes field of the NOAO Deep Wide-Field Survey. We have obtained optical spectroscopic redshifts for 421 sources (71%). These have a redshift distribution peaking at z ~ 0.3, with a possible additional peak at z ~ 0.9, and objects detected out to z = 4.5. The spectra of the remaining 170 (29%) exhibit no strong emission lines from which to determine a redshift. We develop an algorithm to estimate the redshift distribution of these sources, based on the assumption that they have emission lines but that these lines are not observable due to the limited wavelength coverage of our spectroscopic observations. The redshift distribution derived from all 591 sources exhibits an additional peak of extremely luminous (L_(8–1000 μm) > 3 × 10^12 L⊙) objects at z ~ 2, consisting primarily of sources without observable emission lines. We use optical line diagnostics and IRAC colors to estimate that 55% of the sources within this peak are AGN-dominated. We compare our results to published models of the evolution of infrared-luminous galaxies. The models which best reproduce our observations predict a large population of star-formation-dominated ULIRGs at z > 1.5 rather than the AGN-dominated sources we observe.

Additional Information

© 2008 The American Astronomical Society. Received 2007 September 9; accepted 2008 February 18. We appreciate the referee's thoughtful feedback, which led to significant improvements in the paper. We thank Ranga-Ram Chary, David Elbaz, Carlotta Gruppioni, Guilaine Lagache, and Chris Pearson for providing the predictions of their models. We thank Chris Kochanek, Daniel Eisentein, and the AGES Team for providing access to the AGES database. A. D. thanks the Spitzer Science Center for their hospitality while this paper was being written. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology (JPL/Caltech), under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The Spitzer MIPS survey of the Bootes region was obtained using GTO time provided by the Spitzer Infrared Spectrograph Team (PI: James Houck) and by M. Rieke. This work made use of images and data products provided by the NDWFS (Jannuzi & Dey 1999), which is supported by the National Optical Astronomy Observatory (NOAO). NOAO is operated by AURA, Inc., under a cooperative Agreement with the National Science Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck:I (LRIS), Keck:II (DEIMOS), Mayall (MOSAIC-I), MMT (Hectospec), Spitzer (IRAC, IRS, MIPS).

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