Darwin Tames an andromeda dwarf: Unraveling the orbit of NGC 205 using a genetic algorithm

NGC 205, a close satellite of the M31 galaxy, is our nearest example of a dwarf elliptical galaxy. Photometric and kinematic observations suggest NGC 205 is undergoing tidal distortion from its interaction with M31. Despite earlier attempts, the orbit and progenitor properties of NGC 205 are not well known. We perform an optimized search for these unknowns by combining a genetic algorithm with restricted N-body simulations of the interaction. Coupled with photometric and kinematic observations as constraints, this allows for an effective exploration of the parameter space. We represent NGC 205 as a static Hernquist potential with embedded massless test particles serving as tracers of surface brightness. We explore three distinct, initially stable test particle configurations: cold rotating disk, warm rotating disk, and hot, pressure-supported spheroid. Each model reproduces some, but not all, of NGC 205's observed features, leading us to speculate that a rotating progenitor with substantial pressure support could match all of the observables. Furthermore, plausible combinations of mass and scale length for the spheroid progenitor model reproduce the observed velocity dispersion profile. For all three models, orbits best matching the observables place the satellite 11 +/- 9 kpc behind M31 moving at very large velocities: 300-500 km s(-1) on primarily radial orbits. Given that the observed radial component is only 54 km s(-1), this implies a large tangential motion for NGC 205, moving from northwest to southeast. This suggests NGC 205 is not associated with the observed stellar arc northeast of NGC 205. Furthermore, NGC 205's velocity appears near escape velocity, signifying NGC 205 is likely on its first M31 passage.