Proper Motions of Dwarf Spheroidal Galaxies fromHubble Space TelescopeImaging. III. Measurement for Ursa Minor

Abstract

The measured proper motion of Fornax, expressed in the equatorial coordinate system, is (μα,μδ) = (47.6 ± 4.6, - 36.0 ± 4.1) mas century-1. This proper motion is a weighted mean of four independent measurements for three distinct fields. Each measurement uses a quasi-stellar object as a reference point. Removing the contribution of the motion of the Sun and of the local standard of rest to the measured proper motion produces a Galactic rest-frame proper motion of (μ,μ) = (24.4 ± 4.6, - 14.3 ± 4.1) mas century-1. The implied space velocity with respect to the Galactic center has a radial component of Vr = -31.8 ± 1.7 km s-1 and a tangential component of Vt = 196 ± 29 km s-1. Integrating the motion of Fornax in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 118 (66, 137) and 152 (144, 242) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.13 (0.11, 0.38), and the orbital period is 3.2 (2.5, 4.6) Gyr. The orbit is retrograde and inclined by 101° (94°, 107°) to the Galactic plane. Fornax could be a member of a proposed stream of galaxies and globular clusters; however, the membership of another proposed galaxy in the stream, Sculptor, has been previously ruled out. Fornax is in the Kroupa-Theis-Boily plane, which contains 11 of the Galactic satellite galaxies, but its orbit will take it out of that plane.