The flattening of globular clusters: internal rotation or velocity anisotropy?

TL;DR

This study uses new axisymmetric rotating models to analyze the internal dynamics of globular clusters 47 Tuc and omega Cen, revealing how rotation and anisotropy shape their structure and kinematics.

Contribution

It introduces a new family of self-consistent models combining photometry and 3D kinematics to interpret globular cluster dynamics and distances.

Findings

Internal rotation explains 47 Tuc's morphology.

Omega Cen's complex kinematics are well modeled.

Discrepancies in ellipticity linked to anisotropy and rotation.

Abstract

Internal rotation is considered to play a major role in determining the structure and dynamics of some globular clusters. We present a dynamical analysis of the photometry and three-dimensional kinematics of 47 Tuc and omega Cen, by means of a new family of self-consistent axisymmetric rotating models. The combined use of line-of-sight velocities and proper motions allows us to obtain a global description of the internal dynamical structure of the objects together with an estimate of their dynamical distances. The well-relaxed cluster 47 Tuc is very well interpreted by our dynamical models; in particular, internal rotation is found to explain the observed morphology. For the partially relaxed cluster omega Cen, the selected model provides a good representation of its complex three-dimensional kinematics, in general qualitative agreement with the observed anisotropy profile, which is characterized by tangential anisotropy in the outer parts; discrepancies are found between the observed and the expected ellipticity profile and are ascribed to the presence of a high degree of radial anisotropy in the intermediate region and to its interplay with rotation.