PROPER MOTION OF THE SAGITTARIUS DWARF GALAXY BASED ON HUBBLE SPACE TELESCOPE IMAGING

We have derived a proper motion of Sagittarius using archival data obtained with the Hubble Space Telescope. The data consist of imaging at three epochs with a time baseline of about four years in three distinct fields. The zero point for the proper motion is based on the foreground Galactic stellar populations along the line of sight. The measured proper motion in the Galactic coordinate system is (mu(l), mu(b)) = (-2.615 +/- 0.22, 1.87 +/- 0.19) mas yr(-1) and in the equatorial coordinate system is (mu(alpha), mu(delta)) = (-2.75 +/- 0.20, -1.65 +/- 0.22) mas yr(-1). Removing the contribution of the motion of the Sun and of the LSR to the measured proper motion produces a Galactic rest-frame proper motion of (mu(Grf)(l), mu(Grf)(b)) = (-0.82 +/- 0.22, 1.98 +/- 0.19) mas yr(-1) and (mu(Grf)(alpha), mu(Grf)(delta)) = (-2.14 +/- 0.20, 0.03 +/- 0.20) mas yr(-1). The implied space velocity with respect to the Galactic center is (Pi, Theta, Z) = (141.9 +/- 6.9, 117 +/- 29,238 +/- 27) km s(-1). This velocity implies that the instantaneous orbital inclination is 67 degrees, with a 95% confidence interval of (58 degrees, 79 degrees). We also present photometry and membership probabilities for the stars in our sample, which can be used to generate color-magnitude diagrams for stellar populations selected by proper motion.