Kinematics of Stars Along the Sagittarius Trailing Tidal Tail and Constraints on the Milky Way Mass Distribution

摘要

To date, models of the Sagittarius (Sgr) tidal debris streams have been constrained by positions, distances, and radial velocities of detected debris, but no systematic survey has addressed the tangential velocities (derived from proper motions) of Sgr detritus. We present three-dimensional kinematics of 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 (2010) 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 (Θ_(LSR)) 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 3-D model debris kinematics and our measurements into agreement. Satisfactory model fits that simultaneously reproduce known position, distance, and radial velocity trends of the Sgr tidal streams, while significantly increasing Θ_(LSR) 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 (2010).