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

Self-consistent simulations of the dynamical evolution of a low-masssatellite galaxy without dark matter are reported. The orbits haveeccentricities 0.46 <= e <= 0.96 in a Galactic dark halo resembling our MilkyWay. For the simulations, a particle-mesh code with nested sub-grids and adirect-summation N-body code running with the special purpose hardware deviceGRAPE are used. Initially, the satellite is spherical with an isotropicvelocity distribution, and has a mass of 10^7 solar masses. Simulations with1.3x10^5 up to 2x10^6 satellite particles are performed. The calculationsproceed for many orbital periods until well after the satellite disrupts. In all cases the dynamical evolution converges to a remnant that containsroughly 1% of the initial satellite mass. The stable remnant results fromsevere tidal shaping of the initial satellite. To an observer from Earth theseremnants look strikingly similar to the Galactic dwarf spheroidal satellitegalaxies. Their apparent mass-to-light ratios are very large despite the factthat they contain no dark matter. These computations show that a remnant without dark matter displays largerline-of-sight velocity dispersions, sigma, for more eccentric orbits, which isa result of projection onto the observational plane. Assuming they are not darkmatter dominated, it follows that the Galactic dSph satellites with sigma >6km/s should have orbital eccentricities of e > 0.5. Some remnants havesub-structure along the line-of-sight that may be apparent in the morphology ofthe horizontal branch.