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 ^ { \prime } } -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 \mbox { $ { I } $ } { \mbox { $ { z ^ { \prime } } $ } } 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 H _ { 2 } O absorption and a continuum break indicative of CH _ { 4 } , confirming the object has a very cool atmosphere .
Comparison with nearby T dwarfs gives a spectral type of T3–T4 and a distance of \sim 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 2MASS 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 .