Dynamical Constraints on the Dark Matter Distribution of the Sculptor Dwarf Spheroidal from Stellar Proper Motions

We compare the transverse velocity dispersions recently measured within the Sculptor dwarf spheroidal galaxy to the predictions of our previously published dynamical model. This was designed to fit the observed number count and velocity dispersion profiles of both metal-rich and metal-poor stars, both in cored and in cusped potentials. At the projected radius where the proper motions (PMs) were measured, this model (with no change in parameters) predicts transverse dispersions in the range of 6-9.5 km s^-1, with the tangential dispersion about 1 km s^-1 larger than the (projected) radial dispersion. Both dispersions are predicted to be about 1 km s^-1 larger for metal-poor than for metal-rich stars. At this projected radius, cored and cusped potentials predict almost identical transverse dispersions. The measured tangential dispersion (8.5 ± 3.2 km s^-1) agrees remarkably well with these predictions, while the measured radial dispersion (11.5 ± 4.3 km s^-1) differs only at about the 1σ level. Thus, the PM data are in excellent agreement with previous data, but do not help to distinguish between cored and cusped potentials. This will require velocity dispersion data (either from PMs or from radial velocities) with uncertainties well below 1 km s^-1 over a range of projected radii.