The strong rotation of M5 (NGC 5904) as seen from the MIKiS Survey of Galactic Globular Clusters

TL;DR

This study presents detailed measurements of M5's rotation and velocity dispersion, revealing a stable, coherent rotation pattern and significant angular momentum, highlighting the complex kinematic behavior of globular clusters.

Contribution

It provides one of the clearest observations of rotation in a globular cluster, with a stable rotation axis and detailed kinematic profiles, advancing understanding of cluster dynamics.

Findings

M5 exhibits a stable rotation axis at 145° across all radii.

The rotation velocity peaks at ~3 km/s near 0.6 half-mass radii.

Ordered motions are dynamically significant, with rotation contributing notably to the cluster's dynamics.

Abstract

In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters, we present the line-of-sight rotation curve and velocity dispersion profile of M5 (NGC 5904), as determined from the radial velocity of more than 800 individual stars observed out to 700" (~ 5 half-mass radii) from the center. We find one of the cleanest and most coherent rotation patterns ever observed for globular clusters, with a very stable rotation axis (having constant position angle of 145^o at all surveyed radii) and a well-defined rotation curve. The density distribution turns out to be flattened in the direction perpendicular to the rotation axis, with a maximum ellipticity of 0.15. The rotation velocity peak (~3 km/s in projection) is observed at ~0.6 half-mass radii, and its ratio with respect to the central velocity dispersion (~0.3-0.4 at 4 projected half-mass radii) indicates that ordered motions play a significant dynamical role. This result strengthens the growing empirical evidence of the kinematic complexity of Galactic globular clusters and motivates the need of fundamental investigations of the role of angular momentum in collisional stellar dynamics.