Discovery of rotation axis alignments in Milky Way globular clusters

TL;DR

This study reveals a linear relationship between the inclination angles of globular clusters' rotation axes and their orbital axes around the Milky Way, with implications for understanding cluster dynamics and guiding future observations.

Contribution

It uncovers a novel linear correlation between globular clusters' rotation axis inclinations and their orbital inclinations around the Milky Way, independent of other orbital and physical parameters.

Findings

Rotation and orbit axes align at ~56° inclination.

No correlation with orbital size, shape, or internal rotation strength.

Relationship influences future simulations and observational strategies.

Abstract

There is an increasing number of recent observational results which show that some globular clusters exhibit internal rotation while they travel along their orbital trajectories around the Milky Way center. Based on these findings, we looked for any relationship between the inclination angles of the globular clusters' orbits with respect to the Milky Way plane and those of their rotation. We discovered that the relative inclination, in the sense rotation axis inclination - orbit axis inclination, is a function of the globular cluster's orbit inclination. Rotation and orbit axes are aligned for an inclination of ~ 56deg, while the rotation axis inclination is far from the orbit's one between ~ 20deg and -20deg when the latter increases from 0deg up to 90deg. We further investigated the origin of such a linear relationship and found no correlation with the semimajor axes and eccentricities of the globular clusters' orbits, nor with the internal rotation strength, the globular clusters' sizes, actual and tidally disrupted masses, half-mass relaxation times, among others. The uncovered relationship will impact on the development of numerical simulations of the internal rotation of globular clusters, on our understanding about the interaction of the globular clusters with the Milky Way gravitational field, and on the observational campaigns for increasing the number of studied globular clusters with detected internal rotation.