Proper Motion of the Sagittarius Dwarf Galaxy Based on Hubble Space Telescope Imaging

Abstract

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 (μ_ell, μ _b ) = (-2.615 ± 0.22, 1.87 ± 0.19) mas yr^-1 and in the equatorial coordinate system is (μ_α, μ_δ) = (-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 (μ^Grf _ell, μ^Grf _b ) = (-0.82 ± 0.22, 1.98 ± 0.19) mas yr^-1 and (μ^Grf _α, μ^Grf _δ) = (-2.14 ± 0.20, 0.03 ± 0.20) mas yr^-1. The implied space velocity with respect to the Galactic center is (Π, Θ, Z) = (141.9 ± 6.9, 117 ± 29, 238 ± 27) km s^-1. This velocity implies that the instantaneous orbital inclination is 67°, with a 95% confidence interval of (58°, 79°). 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.

Based on observations with NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.