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 μm) 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 H₂O absorption and a continuum break indicative of CH₄, 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 ~45 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. Based on observations obtained at the Subaru Telescope, Infrared Telescope Facility (IRTF), and W. M. Keck Observatory.