Tucana B: A Potentially Isolated and Quenched Ultra-faint Dwarf Galaxy at D ≈ 1.4 Mpc

We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy at a distance of D = 1.4 Mpc. 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 similar to 500 kpc distant. Deep ground-based imaging resolves Tucana B 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. Tucana B has a half-light radius of 80 +/- 40 pc and an absolute magnitude of M-V = -6.9(-0.6)(+0.5) mag (L-V = (5(-2)(+4)) x 10(4) L-circle dot), 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 log(SFRNUV/M circle dot yr(-1)) < 5.4 for star formation on less than or similar to 100 Myr timescales. Given its isolation and physical properties, Tucana B 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.

Automated summary

We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy at a distance of 1.4 Mpc. Tucana B shows a sparse red giant branch consistent with an old, metal-poor stellar population comparable to those in ultra-faint dwarf galaxies of the Milky Way. The absence of young stars suggests that Tucana B may have been quenched by reionization, providing important confirmation of a key driver of galaxy formation and evolution at low mass scales.