Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: IV.: Measurement for sculptor

This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode. Each of two distinct fields contains a quasi-stellar object that serves as the reference point.'' The measured proper motion of Sculptor, expressed in the equatorial coordinate system, is (mu(alpha), mu(delta)) (9 +/- 13; 2 +/- 13) mas century(-1). Removing the contributions from the motion of the Sun and the motion of the local standard of rest produces the proper motion in the Galactic rest frame: (mu(Grf)(alpha), mu(Grf)(delta)) = (-23 +/- 13; 45 +/- 13) mas century(-1). The implied space velocity with respect to the Galactic center has a radial component of V-r = 79 +/- 6 km s(-1) and a tangential component of V-t = 198 +/- 50 km s(-1). Integrating the motion of Sculptor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc, respectively, where the values in the parentheses represent the 95% confidence interval derived from Monte Carlo experiments. The eccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital period is 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the Milky Way: the angle of inclination is 86 degrees (83 degrees, 90 degrees).