Residual velocities of small magellanic cloud star clusters

Abstract

We analyzed the largest Small Magellanic Cloud (SMC) cluster sample (32) with proper motions and radial velocity measurements, from which we obtained their space velocity components. By adopting as a reference the recent best-fitted rotating disc of SMC star clusters as a function of the position angle, we computed their residual velocity vectors, and compared their magnitudes (∆V ) with that of a cluster with residual velocity components equal to the velocity dispersions along the three inde- pendent SMC rotating disc axes of motion (∆V = 60 km/s). We found that clusters that belong to SMC tidally induced structures have ∆V > 50 km/s, which suggests that space velocities of clusters in the process of escaping the rotating disc kinematics, are measurably different. Studied clusters pertaining to a northern branch of the Mag- ellanic Bridge, the main Magellanic Bridge, the Counter-Bridge and the West halo give support to these findings. NGC 121, the oldest known SMC cluster, does not belong to any SMC tidal feature, and has ∆V = 64 km/s, slightly above the boundary between bound and kinematically perturbed clusters.