Ultracool Field Brown Dwarf Candidates Selected at 4.5 microns

Abstract

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 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 AGN. Optical detection of 4 of the remaining 18 sources implies they are likely also AGN, 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, 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. (2003) predict larger numbers of cool brown dwarfs should be found for a Chabrier (2003) mass function. Suppressing the model [4.5] flux by a factor of two, as indicated by previous work, brings the Burrows models and observations into reasonable agreement. The recently launched Wide-field Infrared Survey Explorer (WISE) will probe a volume ~40x larger and should find hundreds of brown dwarfs cooler than T7.

13 pages, 6 figures, accepted for publication in the June 2010 issue of The Astronomical Journal

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• Spitzer bright, UltraVISTA faint sources in COSMOS: the contribution to the overall population of massive galaxies at z=3-7
• The First Ultra-Cool Brown Dwarf Discovered by the Wide-field Infrared Survey Explorer
• The Mid-Infrared Environments of High-Redshift Radio Galaxies
• CFBDSIR J1458+1013B: A Very Cold (>T10) Brown Dwarf in a Binary System
• Confirmation of One of the Coldest Known Brown Dwarfs
• The Ultra Cool Brown Dwarf Companion of WD 0806-661: Age, Mass, and Formation Mechanism
• A UV to Mid-IR Study of AGN Selection
• Spitzer Photometry of WISE-Selected Brown Dwarf and Hyper-Luminous Infrared Galaxy Candidates
• Discovery of Two Very Wide Binaries with Ultracool Companions and a New Brown Dwarf at the L/T Transition
• Hunting the Coolest Dwarfs: Methods and Early Results