A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: VII. Kinematics of the Main Body of the Sagittarius dSph

TL;DR

This study provides a comprehensive spectroscopic analysis of the Sagittarius dwarf galaxy's core, revealing no significant rotation and a flat velocity dispersion profile, which informs our understanding of its dynamical state and tidal disruption.

Contribution

It presents the first large-area, high-resolution spectroscopic survey of the Sagittarius dwarf galaxy's core, offering new insights into its kinematics and lack of intrinsic rotation.

Findings

No significant rotation detected in the Sgr core.

Velocity dispersion remains flat across the galaxy and into tidal tails.

Kinematic properties similar to other Milky Way dwarf spheroidals.

Abstract

We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ~15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 degrees from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high quality spectra we identify 1310 Sgr members out of 2296 stars surveyed distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large area sample shows no evidence for significant rotation, a result at odds with the ~20 km/s rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small (<= 4 km/s) velocity trend along primarily the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.