Dwarf Spheroidal Satellite Galaxies Without Dark Matter: Results From Two Different Numerical Techniques

Abstract

Self-consistent simulations of the dynamical evolution of a low-mass satellite galaxy without dark matter are reported. The orbits have eccentricities 0.46 <= e <= 0.96 in a Galactic dark halo resembling our Milky Way. For the simulations, a particle-mesh code with nested sub-grids and a direct-summation N-body code running with the special purpose hardware device GRAPE are used. Initially, the satellite is spherical with an isotropic velocity distribution, and has a mass of 10^7 solar masses. Simulations with 1.3x10^5 up to 2x10^6 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 look 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, sigma, for more eccentric orbits, which is a result of projection onto the observational plane. Assuming they are not dark matter dominated, it follows that the Galactic dSph satellites with sigma > 6km/s should have orbital eccentricities of e > 0.5. Some remnants have sub-structure along the line-of-sight that may be apparent in the morphology of the horizontal branch.