3D kinematics of SMC star clusters: residual velocities disentangle kinematically perturbed clusters

Abstract

Understanding the kinematic behaviour of the Small Magellanic Cloud (SMC) remains a challenge addressed by many authors using diverse approaches. Over time, increasing observational evidence has accumulated for tidal perturbations induced by the Large Magellanic Cloud (LMC) on the SMC, especially in its outer regions. In this study, we adopt star clusters as kinematic tracers of the SMC. We analyse 36 clusters distributed across the galaxy's structural regions (Northern Bridge, Southern Bridge, Wing/Bridge, West Halo, Main Body and Counter-Bridge). From each cluster's proper motions, radial velocity and heliocentric distance we estimate Cartesian velocities \((V_x,\,V_y,\,V_z)\) in the SMC reference frame. We also compute the same velocity components under the assumption that the SMC behaves as a rotating disc. We then define the residual velocity \(\Delta V\) for each cluster as the difference between the two velocities derived. Additionally, we perform a kinematic anisotropy analysis to characterise the distribution of kinetic energy across the SMC. We find that increasing values of \(\Delta V\) correlate with increasing cluster distance from the SMC center, and that \(\Delta V \approx 60\ \mathrm{km\,s^{-1}}\) it appears to be a lower limit that separates, in kinematic terms, the areas of tidal origin from those with the best behavior.