The core size of the Fornax dwarf Spheroidal

TL;DR

This study uses stellar subpopulations in the Fornax dwarf Spheroidal to directly measure its dark matter density profile, providing strong evidence for a core rather than a cusp, with a core size between 0.6 and 1.8 kpc.

Contribution

It offers the first direct constraints on the core size of Fornax's dark matter halo using multiple stellar populations.

Findings

Fitted a cored dark matter halo to the data

Cored halo is consistent with all stellar populations within 1-sigma

Navarro-Frenk-White halos require unrealistically large scale radii

Abstract

We exploit the detection of three distinct stellar subpopulations in the red giant branch of the Fornax dwarf Spheroidal to probe its density distribution. This allows us to resolve directly the evolution with radius of the dark matter mass profile. We find that a cored dark matter halo provides a perfect fit to the data, being consistent with all three stellar populations well within 1-sigma, and for the first time we are able to put constraints on the core size of such a halo. With respect to previous work, we do not strengthen the statistical exclusion of a dark matter cusp in Fornax, but we find that Navarro-Frenk-White haloes would be required to have unrealistically large scale radii in order to be compatible with the data, hence low values of the concentration parameter. We are then forced to conclude that the Fornax dwarf Spheroidal sits within a dark matter halo having a constant density core, with a core size of between 0.6 and 1.8 kpc.