Magellan IMACS color-magnitude diagram for Tucana B

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.