A curious Milky Way satellite in Ursa Major

In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD) shows a well-defined subgiant branch, main sequence, and turnoff, from which we estimate a distance of similar to 30 kpc and a projected size of similar to 250 x 125 pc(2). The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of, which would make it the faintest known satellite galaxy. UMa II's isophotes are irregular M-V similar to -3.8 and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.