The Discovery of an Ultra-Faint Star Cluster in the Constellation of
  Ursa Minor
  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 (CFHT). We find that this cluster,
Munoz 1, is located at a distance of 45 +/- 5 kpc and at a projected distance
of only 45 arcmin from the center of the Ursa Minor dSph galaxy. Using a
Maximum Likelihood technique we measure a half-light radius of 0.5 arcmin, 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_sun 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 Munoz 1
is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the
faintest stellar clusters known to date.
