Measuring the Dark Matter Scale of Local Group Dwarf Spheroidals

Abstract

We reanalyze published Draco and Ursa Minor dwarf spheroidal (dSph) stellar velocity data. Previous studies of dSph stellar distributions have concentrated principally on the spatial distribution of stars or on computation of a global velocity dispersion. A detailed understanding of the spatial behavior of the velocity distribution, however, is necessary to distinguish among competing theories of dSph dynamics. Here we apply a maximum-likelihood technique to fit the set of individual velocity measurements to a global distribution. Rather than calculating mean local velocity dispersions in regions where the data are sparse, this technique has the advantage of using the velocity measurements individually. We confirm earlier findings of a radial falloff in the velocity distribution of UMi, which is consistent with a mass-follows-light (MFL) King model. We also confirm an apparent radial rise in the velocity dispersion of Draco. However, these data do not suffice to distinguish among extended halo and MFL models. Finally, we perform simulations to determine the number of additional observations required to clearly differentiate between different dynamical models. Only ~10–20 additional observations at 0.75 times the tidal radius would be required to distinguish clearly between an MFL distribution and an extended halo or disrupted remnant model with a flat or radially rising velocity dispersion. We describe a method for treating the effect of binary stars, which may be a source of contamination.