We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy at a distance of D=1.4Mpc. Tucana B was found during a search for ultra-faint satellite companions to the known dwarfs in the outskirts of the Local Group, although its sky position and distance indicate the nearest galaxy to be ~500kpc distant. Deep ground-based imaging resolves TucanaB into stars, and it displays a sparse red giant branch consistent with an old, metal-poor stellar population analogous to that seen in the ultra-faint dwarf galaxies of the Milky Way, albeit at fainter apparent magnitudes. TucanaB has a half-light radius of 80{+/-}40pc and an absolute magnitude of M_V_=-6.9_-0.6_^+0.5^mag (L_V_=(5_-2_^+4^)x10^4^L{sun}), which is again comparable to the Milky Way's ultra-faint satellites. There is no evidence for a population of young stars, either in the optical color-magnitude diagram or in GALEX archival ultraviolet imaging, with the GALEX data indicating logSFR_NUV_}/M{odot}/yr)<-5.4 for star formation on ~100Myr timescales. Given its isolation and physical properties, TucanaB may be a definitive example of an ultra-faint dwarf that has been quenched by reionization, providing strong confirmation of a key driver of galaxy formation and evolution at the lowest mass scales. It also signals a new era of ultra-faint dwarf galaxy discovery at the extreme edges of the Local Group.

Cone search capability for table J/ApJ/935/L17/fig2 (The color-magnitude diagram of Tucana B from our Magellan IMACS data)