Space Motions of the Dwarf Spheroidal Galaxies Draco and Sculptor Based on HST Proper Motions with a ∼10 yr Time Baseline

We present new proper motion (PM) measurements of the dwarf spheroidal galaxies (dSphs) Draco and Sculptor using multiepoch images obtained with the Hubble Space Telescope ACS/WFC. Our PM results have uncertainties far lower than previous measurements, even those made with the same instrument. The PM results for Draco and Sculptor are (μ_w,μ_N)_Dra = (-0.562 ± 0.0099, -0.1765 ± 0.0100) mas yr^-1 and (μ_w,μ_N)_Scl = (-0.296 ± 0.0209, -0.1358 ± 0.0214) mas yr^-1. The implied Galactocentric velocity vectors for Draco and Sculptor have radial and tangential components: (V_rad, V_tan)_Dra = (-88.6, 161.4) ± 4.4, 5.6 km s^-1 and (V_rad, V_tan)_Scl = (72.6, 200.2) ± (1.3, 10.8) km s^-1. We study the detailed orbital histories of both Draco and Sculptor via numerical orbit integrations. Orbital periods of Draco and Sculptor are found to be 1-2 Gyr and 2-5 Gyr, respectively, accounting for uncertainties in the Milky Way (MW) mass. We also study the influence of the Large Magellanic Cloud (LMC) on the orbits of Draco and Sculptor. Overall, the inclusion of the LMC increases the scatter in the orbital results. Based on our calculations, Draco shows a rather wide range of orbital parameters depending on the MW mass and inclusion/exclusion of the LMC, but Sculptor's orbit is very well constrained, with its most recent pericentric approach to the MW being 0.3-0.4 Gyr ago. Our new PMs imply that the orbital trajectories of both Draco and Sculptor are confined within the "Disk of Satellites," better so than implied by earlier PM measurements, and likely rule out the possibility that these two galaxies were accreted together as part of a tightly bound group.