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 r _ { 0 } = 1 ^ { +0.8 } _ { -0.4 } kpc .