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Published June 2010 | Published
Journal Article Open

Ultracool Field Brown Dwarf Candidates Selected at 4.5 μm


We have identified a sample of cool field brown dwarf candidates using IRAC data from the Spitzer Deep, Wide-Field Survey (SDWFS). The candidates were selected from 400,000 SDWFS sources with [4.5] ≤ 18.5 mag and were required to have [3.6] – [4.5] ≥ 1.5 and [4.5] – [8.0] ≤ 2.0 on the Vega system. The first color requirement selects objects redder than all but a handful of presently known brown dwarfs with spectral classes later than T7, while the second eliminates 14 probable reddened active galactic nuclei (AGNs). Optical detection of four of the remaining 18 sources implies they are likely also AGNs, leaving 14 brown dwarf candidates. For two of the brightest candidates (SDWFS J143524.44+335334.6 and SDWFS J143222.82+323746.5), the spectral energy distributions including near-infrared detections suggest a spectral class of ~T8. The proper motion is <0".25 yr^(–1), consistent with expectations for a luminosity-inferred distance of >70 pc. The reddest brown dwarf candidate (SDWFS J143356.62+351849.2) has [3.6] – [4.5] = 2.24 and H – [4.5] > 5.7, redder than any published brown dwarf in these colors, and may be the first example of the elusive Y-dwarf spectral class. Models from Burrows et al. predict that larger numbers of cool brown dwarfs should be found for a Chabrier mass function. Suppressing the model [4.5] flux by a factor of 2, as indicated by previous work, brings the Burrows models and observations into reasonable agreement. The recently launched Wide-field Infrared Survey Explorer will probe a volume ~40× larger and should find hundreds of brown dwarfs cooler than T7.

Additional Information

© 2010 The American Astronomical Society. Received 2009 July 27; accepted 2010 April 7; published 2010 May 3. The authors thank Emanuele Daddi, Mark Dickinson, Jason Melbourne, and Tom Soifer for assistance obtaining observations; Nick Seymour for assistance with SDWFS and WIRC reductions; and Vandana Desai for the Mrk 231 spectrum and information about DOG SED's. Tom Soifer, Marcia Rieke, Dan Weedman, and Jim Houck are thanked for allowing access to the GTO MIPS survey of the NDWFS, and we acknowledge Buell Jannuzi's central role in the NDWFS and related surveys of the field. Discussions with Roc Cutri helped us understand the mid-IR characteristics of AGB stars, and Szymon Kozlowski clarified questions about SDWFS variability measurements. We thank the anonymous referee for a detailed and careful review which improved the accuracy of the presentation. 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 contract with NASA. This work made use of images and data products provided by the NOAO Deep Wide-Field Survey (NDWFS), which is supported by the National Optical Astronomy Observatory (NOAO), and followup NOAO surveys. NOAO is operated by AURA, Inc., under a cooperative agreement with the National Science Foundation. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among Caltech, the University of California and NASA. The Keck Observatory was made possible by the generous financial support of the W. M. Keck Foundation, which also provided support for M.B. Some data were obtained at the Hale Telescope, Palomar Observatory as part of a continuing collaboration between Caltech, NASA/JPL, and Cornell University. Support for this work was provided by NASA through an award issued by JPL/Caltech. A.H.G. acknowledges support for this work by the National Science Foundation (NSF) under grant 0708490. Support for A.M.G. and Q.K.'s contribution to this work was provided by the NSF Science & Technology Center for AO, managed by UCSC (AST-9876783), and the Levine-Leichtman Family Foundation. Facilities: Spitzer (IRAC, MIPS), Hale (WIRC), Keck:II (NIRC2), Mayall (NEWFIRM)

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