Kinematics and Chemistry of Stars along the Sagittarius Trailing Tidal Tail and Constraints on the Milky Way Mass Distribution
We present three-dimensional (3D) kinematics of Sagittarius (Sgr) trailing tidal debris in six fields located 70°-130° along the stream from the Sgr dwarf galaxy core. The data are from our proper-motion (PM) survey of Kapteyn's Selected Areas, in which we have measured accurate PMs to faint magnitudes in ~40' × 40' fields evenly spaced across the sky. The radial velocity (RV) signature of Sgr has been identified among our follow-up spectroscopic data in four of the six fields and combined with mean PMs of spectroscopically confirmed members to derive space motions of Sgr debris based on ~15-64 confirmed stream members per field. These kinematics are compared to predictions of the Law & Majewski model of Sgr disruption; we find reasonable agreement with model predictions in RVs and PMs along Galactic latitude. However, an upward adjustment of the local standard of rest velocity (ΘLSR) from its standard 220 km s–1 to at least 232 ± 14 km s–1 (and possibly as high as 264 ± 23 km s–1) is necessary to bring 3D model debris kinematics and our measurements into agreement. Satisfactory model fits that simultaneously reproduce known position, distance, and RV trends of the Sgr tidal streams, while significantly increasing ΘLSR, could only be achieved by increasing the Galactic bulge and disk mass while leaving the dark matter halo fixed to the best-fit values from Law & Majewski. We derive low-resolution spectroscopic abundances along this stretch of the Sgr stream and find a constant [Fe/H] ~ –1.15 (with ~0.5 dex scatter in each field—typical for dwarf galaxy populations) among the four fields with reliable measurements. A constant metallicity suggests that debris along the ~60° span of this study was all stripped from Sgr on the same orbital passage.