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 which serves as the “ reference point. ” The measured proper motion of Sculptor , expressed in the equatorial coordinate system , is ( \mu _ { \alpha } , \mu _ { \delta } ) = ( 9 \pm 13 , 2 \pm 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 _ { \alpha } ^ { \mbox { \tiny { Grf } } } , \mu _ { \delta } ^ { \mbox { \tiny { Grf } } } ) = ( -23 \pm 13 % , 45 \pm 13 ) mas century ^ { -1 } .
The implied space velocity with respect to the Galactic center has a radial component of V _ { r } = 79 \pm 6 km s ^ { -1 } and a tangential component of V _ { t } = 198 \pm 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 ) kpc 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 ~ { } ( 83 , 90 ) degrees .