On the rotation of nuclear star clusters formed by cluster inspirals

Nuclear star clusters (NSCs) are commonly observed in the centres of most galactic nuclei, including our own Milky Way (MW). 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 an MW-like nuclear bulge that hosts a massive black hole. We find that the NSC formed through this process can show both morphological and kinematical properties that make it comparable with observations of the MW NSC, including significant rotation - a fact that has so far been 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.