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

TL;DR

This study measures the 3D kinematics of Sagittarius stream stars to refine models of the Milky Way's mass distribution, revealing the need to adjust the Local Standard of Rest velocity and suggesting a constant metallicity along the stream.

Contribution

It provides new proper motion and radial velocity data for Sagittarius stream stars, constrains the Milky Way's mass distribution, and suggests a higher LSR velocity than standard.

Findings

Reasonable agreement with the Law & Majewski (2010) model in RVs and PMs.

An upward adjustment of Theta_LSR to at least 232 km/s improves model-data fit.

Constant [Fe/H] = -1.15 along the stream indicates a common orbital passage.

Abstract

We present three-dimensional kinematics of Sagittarius (Sgr) trailing tidal debris in six fields located 70-130 degrees 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 40x40 arcmin 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 Theta_LSR from its standard 220 km/s to at least $232\pm14$ km/s (and possibly as high as $264\pm23$ km/s) 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 Theta_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). 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-degree span of this study was all stripped from Sgr on the same orbital passage.