Early dynamical evolution of rotating star clusters in a tidal field

TL;DR

This study uses N-body simulations to investigate how the early rotational properties of star clusters evolve under external tidal influences, revealing complex internal kinematics and structural changes.

Contribution

It provides new insights into the impact of tidal fields on the internal rotation and morphology of star clusters during their early evolution.

Findings

Complex kinematic structures develop due to external torque.

Projected anisotropy can be influenced by rotation axis variation.

Clusters may become triaxial with misaligned axes.

Abstract

In order to explore how the early internal rotational properties of star clusters are affected by the external potential of their host galaxies, we have run a suite of $N$-body simulations following the early dynamical evolution and violent relaxation of rotating star clusters embedded in a tidal field. Our study focuses on models for which the cluster's rotation axis has a generic orientation relative to the torque of the tidal field. The interaction between the violent relaxation process, angular momentum of the cluster, and the external torque creates a complex kinematic structure within the cluster, most prominently a radial variation in the position of the rotation axis along both the polar and azimuthal directions. We also examine the cluster's velocity dispersion anisotropy and show that the projected anisotropy may be affected by the variation of the rotation axis directions within the cluster; the combination of projection effects and the complex kinematical features may result in the measurement of tangential anisotropy in the cluster's inner regions. We also characterize the structural properties of our clusters as a function of their initial rotation and virial ratio and find that clusters may develop a triaxial morphology and a radial variation of the minor axis not necessarily aligned with the rotation axis. Finally, we examine the long-term evolution of these complex kinematic features.