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] ≤ 18.5mag 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 (SDWFSJ143524.44+335334.6 and SDWFSJ143222.82+323746.5), the spectral energy distributions including near-infrared detections suggest a spectral class of T8. The proper motion is <025yr^-1, consistent with expectations for a luminosity-inferred distance of >70 pc. The reddest brown dwarf candidate (SDWFSJ143356.62+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 etal. 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.