Dwarf Spheroidal Satellite Galaxies without Dark Matter: Results from Two Different Numerical Techniques

Self-consistent simulations of the dynamical evolution of a low-mass satellite galaxy without dark matter are reported. The orbits have eccentricities of 0.41 <= e <= 0.96 in a Galactic dark halo with a mass of 2.85 × 10¹² M_⊙ and 4.5 × 10¹¹ M_⊙. A particle-mesh code with nested subgrids and a direct-summation N-body code running with the special-purpose hardware device GRAPE are used for the simulations. Initially, the satellite is spherical with an isotropic velocity distribution and a mass of 10⁷ M_⊙. Simulations with 1.3 × 10⁵ up to 2 × 10⁶ satellite particles are performed. The calculations proceed for many orbital periods, until well after the satellite disrupts. In all cases, the dynamical evolution converges to a remnant that contains roughly 1% of the initial satellite mass. The stable remnant results from severe tidal shaping of the initial satellite. To an observer from Earth, these remnants appear strikingly similar to the Galactic dwarf spheroidal satellite galaxies. Their apparent mass-to-light ratios are very large, despite the fact that they contain no dark matter. These computations show that a remnant without dark matter displays larger line-of-sight velocity dispersions, σ, for more eccentric orbits, as a result of projection onto the observational plane. Assuming that they are not dark matter dominated, it follows that the Galactic dSph satellites with σ > 6 km s^-1 should have orbital eccentricities of e > 0.5. Some remnants have substructure along the line of sight that may be apparent in the morphology of the horizontal branch.