Two kinematically distinct old globular cluster populations in the Large Magellanic Cloud

Abstract

We report results of proper motions of 15 known Large Magellanic Cloud (LMC) oldglobular clusters (GCs) derived from the Gaia DR2 data sets. When these mean propermotions are gathered with existent radial velocity measurements to compose the GCs´ velocity vectors, we found that the projection of the velocity vectors onto the LMC plane and those perpendicular to it tell us about two distinct kinematic GCpopulations. Such a distinction becomes clear if the GCs are split at a perpendicularvelocity of 10 km/s (absolute value). The two different kinematic groups also exhibitdifferent spatial distributions. Those with smaller vertical velocities are part of theLMC disc, while those with larger values are closely distributed like a sphericalcomponent. Since GCs in both kinematic-structural components share similar ages andmetallicities, we speculate with the possibility that their origins could have occurredthrough a fast collapse that formed halo and disc concurrently.