Dynamical constraints on the dark matter distribution of the Sculptor dwarf spheroidal from stellar proper motions

TL;DR

This study compares observed stellar proper motions in the Sculptor dwarf galaxy with dynamical models, finding good agreement but limited ability to distinguish between different dark matter potential profiles.

Contribution

It provides a detailed comparison of proper motion data with models, highlighting the need for more precise measurements to differentiate dark matter distributions.

Findings

Proper motion data agree with existing models.

Current data cannot distinguish between cored and cusped potentials.

More precise velocity dispersion measurements are needed.

Abstract

We compare the transverse velocity dispersions recently measured within the Sculptor dwarf spheroidal galaxy to the predictions of previously published dynamical models. These provide good fits to the observed number count and velocity dispersion profiles of metal-rich and metal-poor stars both in cored and in cusped potentials. At the projected radius where the proper motions were measured, these models predict transverse dispersions in the range 6 to 9.5 km/s, with the tangential dispersion about 1 km/s larger than the (projected) radial dispersion. Both dispersions are predicted to be about 1 km/s 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 ($8.5 \pm 3.2$ km/s) agrees remarkably well with these predictions, while the measured radial dispersion ($11.5 \pm 4.3$ km/s) differs only at about the $1\sigma$ level. Thus, the proper motion 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 proper motions or from radial velocities) with uncertainties well below 1 km/s over a range of projected radii.