The core size of the Fornax dwarf spheroidal

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 of the dark matter mass profile with radius. We find that a cored dark matter halo is consistent well within 1 sigma with all three stellar populations, 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 r(0) = 1(-0.4)(+0.8) kpc.