Rotation in young massive star clusters

Hydrodynamical simulations of turbulent molecular clouds show that star clusters form from the hierarchical merger of several sub-clumps. We run smoothed-particle hydrodynamics simulations of turbulence-supported molecular clouds with mass ranging from 1700 to 43 000 M. We study the kinematic evolution of the main cluster that forms in each cloud. We find that the parent gas acquires significant rotation, because of large-scale torques during the process of hierarchical assembly. The stellar component of the embedded star cluster inherits the rotation signature from the parent gas. Only star clusters with final mass < few × 100 M do not show any clear indication of rotation. Our simulated star clusters have high ellipticity (~0.4-0.5 at t = 4 Myr) and are subvirial (Qvir 0.4). The signature of rotation is stronger than radial motions due to subvirial collapse. Our results suggest that rotation is common in embedded massive (1000 M) star clusters. This might provide a key observational test for the hierarchical assembly scenario.