Kinematical fingerprints of star cluster early dynamical evolution

TL;DR

This study investigates how external tidal fields influence the early dynamical evolution of star clusters, revealing that they develop differential rotation and specific anisotropy profiles due to symmetry breaking and Coriolis effects during violent relaxation.

Contribution

It demonstrates that external tidal fields induce distinctive kinematical features in star clusters, such as differential rotation and anisotropy variations, which serve as fingerprints of their early dynamical history.

Findings

Clusters develop differential rotation with a peak between 1-2 half-mass radii.

Radial anisotropy peaks in intermediate regions and decreases outward.

Kinematical fingerprints can persist in young or less-relaxed clusters.

Abstract

We study the effects of the external tidal field on the violent relaxation phase of star clusters dynamical evolution, with particular attention to the kinematical properties of the equilibrium configurations emerging at the end of this phase.We show that star clusters undergoing the process of violent relaxation in the tidal field of their host galaxy can acquire significant internal differential rotation and are characterized by a distinctive radial variation of the velocity anisotropy. These kinematical properties are the result of the symmetry breaking introduced by the external tidal field in the collapse phase and of the action of the Coriolis force on the orbit of the stars. The resulting equilibrium configurations are characterized by differential rotation, with a peak located between one and two half-mass radii. As for the anisotropy, similar to clusters evolving in isolation, the systems explored in this Letter are characterized by an inner isotropic core, followed by a region of increasing radial anisotropy. However for systems evolving in an external tidal field the degree of radial anisotropy reaches a maximum in the cluster intermediate regions and then progressively decreases, with the cluster outermost regions being characterized by isotropy or a mild tangential anisotropy. Young or old but less-relaxed dynamically young star clusters may keep memory of these kinematical fingerprints of their early dynamical evolution.