On the rotation of nuclear star clusters formed by cluster-inspirals

TL;DR

This paper demonstrates through simulations that cluster-inspiral can produce rotating nuclear star clusters with properties similar to those observed, challenging the idea that gas infall is the main cause of rotation.

Contribution

It provides evidence that cluster-inspiral alone can account for the rotation and properties of nuclear star clusters, without requiring fine-tuned orientations.

Findings

NSCs formed by cluster-inspiral can exhibit significant rotation.

Rotation in NSCs does not require fine-tuned infall orientations.

Cluster-inspiral is a viable formation mechanism for rotating NSCs.

Abstract

Nuclear Star Clusters (NSCs) are commonly observed in the centres of most galactic nuclei, including our own Milky Way. While their study can reveal important information about the build-up of the innermost regions of galaxies, the physical processes that regulate their formation are still poorly understood. NSCs might have been formed through gas infall and subsequent in situ star formation, and/or through the infall and merging of multiple star clusters into the centre of the galaxy. Here, we investigate the viability of the latter, by studying direct N-body simulations of inspiralling clusters to the centre of a Milky-Way-like nuclear bulge that hosts a massive black hole. We find that the NSC that forms through this process can show both morphological and kinematical properties that make it comparable with observations of the Milky Way NSC, including significant rotation- a fact that has been so far attributed mainly to gas infall. We explore its kinematic evolution, to see if and how the merger history can imprint fossil records on its dynamical structure. Moreover, we study the effect of stellar foreground contamination in the line-of-sight kinematics of the NSC. Our study shows that no fine tuning of the orientation of the infalling globular clusters is necessary to result in a rotating NSC. We suggest that cluster-inspiral is a viable mechanism for the formation of rotating NSCs.