Schwarzschild dynamical model of the Fornax dwarf spheroidal galaxy

We present a full dynamical model of the Fornax dwarf spheroidal galaxy obtained with the spherically symmetric Schwarzschild orbit superposition method applied to the largest kinematic data set presently available. We modelled the total mass content of the dwarf with the mass-to-light ratio Upsilon varying with radius and found that Fornax is with high probability embedded in a massive and extended dark matter halo. We estimated the total mass contained within 1 kpc to be M(< 1 kpc) = 1.25(0.13)(+0.06) x 10(8) M-circle dot. The data are consistent with the constant mass-to-light ratio, i.e. the mass-follows-light model, only at 3 sigma level, but still require a high value of Upsilon approximate to 11.2 M-circle dot/L-circle dot. Our results are in general agreement with previous estimates of the dynamical mass profile of Fornax. As the Schwarzschild method does not require any assumptions on the orbital anisotropy of the stars, we obtained a profile of the anisotropy parameter beta as an output of our modelling. The derived anisotropy is close to zero in the centre of the galaxy and decreases with radius, but remains consistent with isotropic orbits at all radii at 1 sigma confidence level.