| Abstract | 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 ± 5kpc 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 05, or equivalently 7pc, 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 ± 4kms -1 based on Keck/DEIMOS spectroscopy. This new satellite is separate from Ursa Minor by 30kpc and 110km s -1 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. © 2012. The American Astronomical Society. All rights reserved.. |
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