The ESO Multi-instrument Kinematic Survey (MIKiS) of Galactic Globular Clusters: Solid-body Rotation and Anomalous Velocity Dispersion Profile in NGC 5986 ∗

Abstract

As part of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters (GCs), we present a detailed investigation of the internal kinematics of NGC 5986. The analysis is based on about 300 individual radial velocities of stars located at various distances from the cluster center, up to 300″ (about four half-mass radii). Our analysis reveals the presence of a solid-body rotation extending from the cluster center to the outermost regions probed by the data, and a velocity dispersion profile initially declining with the distance from the cluster’s center, but flattening and staying constant at ∼5 km s −1 for distances larger than about one half-mass radius. This is the first GC for which evidence of the joint presence of solid-body rotation and flattening in the outer velocity dispersion profile has been found. The combination of these two kinematical features provides a unique opportunity to shed light on fundamental aspects of GC dynamics and to probe the extent to which internal relaxation, star escape, angular momentum transport and loss, and the interaction with the Galaxy tidal field can affect a cluster’s dynamical evolution and determine its current kinematical properties. We present the results of a series of N -body simulations illustrating the possible dynamical paths leading to kinematic features like those observed in this cluster and the fundamental dynamical processes that underpin them.