The mass distribution of dwarf spheroidal galaxies from stellar kinematics: Draco, Ursa Minor and Fornax

We model three dSph galaxies, Draco, Ursa Minor and Fornax, as axisymmetric stellar systems embedded in spherical dark-matter potentials, which are in dynamical equilibrium without significant external tidal forces. We construct non-parametric twoand three-integral models that match the observed surfacedensity profiles and the current kinematic samples of∼ 150−200 stars per galaxy; these models naturally produce the so-called “extra-tidal extensions”, which had previously been suggested as evidence of tidal stripping. Isochrone, NFW and power-law models fit the data, but we strongly rule out any centrally condensed mass distribution like a dominant central black hole, as well as constant-density and (for Draco and Fornax) constant mass-to-light ratio models. The average Vband mass-to-light ratio is 400± 80 M⊙/LV,⊙ within 0.75 kpc for Draco, 580 −100 M⊙/LV,⊙ within 1.1 kpc for Ursa Minor and 25 +7 −5 M⊙/LV,⊙ within 2.5 kpc for Fornax. Two-integral models fit the data almost as well as three-integral models; in contrast to previous suggestions we do not find that anisotropy contributes substantially to the high mass-to-light ratios in these dSph galaxies. Subject headings: Methods: statistical – galaxies: kinematics and dynamics – galaxies: fundamental parameters – galaxies: individual (Draco, Ursa Minor, Fornax) – cosmology: dark matter