Confirmation of one of the coldest known brown dwarfs

Using two epochs of 4.5 μm images from the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope, we recently identified a common proper motion companion to the white dwarf WD0806-661 that is a candidate for the coldest known brown dwarf. To verify its cool nature, we have obtained images of this object at 3.6 μm with IRAC, at J with the High Acuity Wide-field K-band Imager (HAWK-I) on the Very Large Telescope, and in a filter covering the red half of J with FourStar on Magellan. WD0806-661B is detected by IRAC but not HAWK-I or FourStar. From these data we measure colors of [3.6] - [4.5] = 2.77 0.16 and J - [4.5] > 7.0 (S/N < 3). Based on these colors and its absolute magnitudes, WD0806-661B is the coldest companion directly imaged outside of the solar system and is a contender for the coldest known brown dwarf with the Y dwarf WISEPJ1828+2650. It is unclear which of these two objects is colder given the available data. A comparison of its absolute magnitude at 4.5 μm to the predictions of theoretical spectra and evolutionary models suggests that WD0806-661B has T _eff = 300-345K.