Discovery of a methane dwarf from the IfA Deep Survey

We present the discovery of a distant methane dwarf, the first from the Institute for Astronomy (IfA) Deep Survey. The object ("IfA 0230-Z1") was identified from deep optical I- and z'-band imaging, being conducted as an IfA-wide collaboration using the prime-focus imager Suprime-Cam on the Subaru 8.2 m Telescope. IfA 0230-Z1 is extremely red in the Iz'J(0.8-1.2 mum) bands but relatively blue in J-H; such colors are uniquely characteristic of T dwarfs. A near-IR spectrum taken with the Keck Telescope shows strong H(2)O absorption and a continuum break indicative of CH(4), confirming that the object has a very cool atmosphere. Comparison with nearby T dwarfs gives a spectral type of T3-T4 and a distance of similar to45 pc. Simple estimates based on previous T dwarf discoveries suggest that the IfA survey will find a comparable number of T dwarfs as the 2 Micron All Sky Survey, albeit at a much larger average distance. We also discuss the survey's ability to probe the Galactic scale height of ultracool (L and T) dwarfs.