2012ApJ...753L..15M

We report the discovery of a new ultra-faint globular cluster in the constellation of Ursa Minor, based on stellar photometry from the MegaCam imager at the Canada-France-Hawaii Telescope. We find that this cluster, Muñoz 1, is located at a distance of 45±5 kpc and at a projected distance of only 45' from the center of the Ursa Minor dwarf spheroidal galaxy. Using a maximum-likelihood technique we measure a half-light radius of 0'.5, or equivalently 7 pc, and an ellipticity consistent with being zero. We estimate its absolute magnitude to be M_V= -0.4±0.9, which corresponds to L_V = 120⁺¹⁶⁰_- 65_L☉_ and we measure a heliocentric radial velocity of -137±4 km/s based on Keck/DEIMOS spectroscopy. This new satellite is separate from Ursa Minor by ∼30 kpc and 110 km/s suggesting the cluster is not obviously associated with the dSph, despite the very close angular separation. Based on its photometric properties and structural parameters we conclude that Muñoz 1 is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the faintest stellar clusters known to date.