Central rotations of Milky Way Globular Clusters

TL;DR

This study reveals that most Milky Way globular clusters exhibit central rotation, which correlates with their flattening and velocity dispersion, suggesting rotation as a key factor in their shape and dynamics.

Contribution

The paper provides the first comprehensive survey of central rotation in Milky Way GCs using integral field spectroscopy, confirming rotation in all observed clusters and linking it to their flattening.

Findings

All 11 observed GCs show central rotation.

Strong correlation between central rotation and outer ellipticity.

Rotation influences the measured central velocity dispersion.

Abstract

Most Milky Way globular clusters (GCs) exhibit measurable flattening, even if on a very low level. Both cluster rotation and tidal fields are thought to cause this flattening. Nevertheless, rotation has only been confirmed in a handful of GCs, based mostly on individual radial velocities at large radii. We are conducting a survey of the central kinematics of Galactic GCs using the new Integral Field Unit instrument VIRUS-W. We detect rotation in all 11 GCs that we have observed so far, rendering it likely that a large majority of the Milky Way GCs rotate. We use published catalogs of the ACS survey of GCs to derive central ellipticities and position angles. We show that in all cases where the central ellipticity permits an accurate measurement of the position angle, those angles are in excellent agreement with the kinematic position angles that we derive from the VIRUS-W velocity fields. We find an unexpected tight correlation between central rotation and outer ellipticity, indicating that rotation drives flattening for the objects in our sample. We also find a tight correlation between central rotation and published values for the central velocity dispersion, most likely due to rotation impacting the old dispersion measurements.