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 sub-giant branch , main sequence and turn-off , from which we estimate a distance of \sim 30 kpc and a projected size of \sim 250 \times 125 pc .
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 M _ { V } \sim - 3.8 , which would make it the faintest known satellite galaxy .
UMa II ’ s isophotes are irregular and distorted with evidence for multiple concentrations ; this suggests that the satellite is in the process of disruption .