Keywords

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_ℓ, 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_ℓ, 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.

We derive the distance to the northern extension of the Sagittarius (Sgr) dwarf spheroidal galaxy from 203 Sgr RR0 Lyrae stars found in the MACHO database. Their distances are determined differentially with respect to 288 Galactic bulge RR0 Lyrae stars also found in the MACHO data. We find a distance modulus difference of 2.41 mag at l = 5° and b = −8° and that the extension of the Sgr galaxy toward the galactic plane is inclined toward us. Assuming R_GC = 8 kpc, this implies the distance to these stars is (m − M)₀ = 16.97 ±  0.07 mag, which corresponds to D = 24.8 ± 0.8 kpc. Although this estimate is smaller than previous determinations for this galaxy and agrees with previous suggestions that Sgr's body is truly closer to us, this estimate is larger than studies at comparable galactic latitudes.