The kinematics of Small Magellanic Cloud star clusters

Abstract

We report results of the proper motions of 25 known Small Magellanic Cloud (SMC) clusters (ages ∼1-10 Gyr old) derived from Gaia EDR3 data sets. When these mean proper motions were gathered with existent radial velocity measurements to compose the clusters' velocity vectors, we found the parameter values of a rotation disk that best resemble their observed motions, namely: central coordinates and distance, inclination and position angle of the line-of-nodes, proper motion in right ascension and declination, systemic velocity, rotation velocity, and velocity dispersion. The SMC cluster rotation disk seems to be, at some level, kinematically synchronized with the rotation of the field red giants recently modeled using DR2 data sets. Such a rotation disk is seen in the sky as a tilted edge-on disk, with a velocity dispersion perpendicular to it that is twice as large as that in the plane of the disk. Because the direction perpendicular to the disk is nearly aligned with the Magellanic Bridge, we interpret the larger velocity dispersion as a consequence of the SMC velocity stretching caused by the tidal interaction with the Large Magellanic Cloud. Rotation alone does not seem sufficient to explain the observed kinematic behaviors in the SMC.