Exploring halo substructure with giant stars: The velocity dispersion profiles of the Ursa Minor and Draco dwarf spheroidal galaxies at large angular separations

We analyze velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal (dSph) galaxies based on published and new Keck HIRES spectra for stars in the outer UMi field. Washington + DDO51 filter photometric catalogs provide additional leverage on membership of individual stars and, beyond 0.5 King limiting radii (r_lim), identify bona fide dSph members up to 4.5 times more efficiently than simple color-magnitude diagram selections. Previously reported "cold populations" at r_lim are not obvious in the data and appear only with particular binning; more or less constant and platykurtic dispersion profiles are characteristic of these dSphs to large radii. We report the discovery of UMi stars to at least 2.7r _lim (i.e., 210′ or 4 kpc). Even with conservative assumptions, a UMi mass of M > 4.9 × 10⁸ M⊙ is required to bind these stars, implying an unlikely global mass-to-light ratio of M/L > 900 (M/L)⊙. We conclude that we have found stars tidally stripped from UMi.