Dynamical models for the Sculptor dwarf spheroidal in a Lambda CDM universe

Abstract

The Sculptor dwarf spheroidal galaxy contains two distinct stellar populations: one metal-rich and the other metal-poor. Several authors have argued that in order for these two populations to reside in the same gravitational potential, the dark matter halo must have a core similar to that observed in the stellar count profile. This would rule out the cuspy Navarro-Frenk-White (NFW) density profiles predicted for halos and subhalos by dark matter only simulations of the Lambda Cold Dark Matter (Lambda-CDM) cosmological model. We present a new theoretical framework to analyse stellar count and velocity observations in a self-consistent manner based on separable models, f(E,J)=g(J)h(E), for the distribution function of an equilibrium spherical system. We use this machinery to analyse available photometric and kinematic data for the two stellar populations in Sculptor. We find, contrary to some previous claims, that the data are consistent with populations in equilibrium within an NFW dark matter potential with structural parameters in the range expected in Lambda-CDM. Our solution gives a maximum circular velocity for Sculptor between 20 and 35 km/s. We discuss why some previous authors came to a different conclusion.