The kinematics of Small Magellanic Cloud star clusters

TL;DR

This study analyzes the proper motions of 25 Small Magellanic Cloud star clusters using Gaia EDR3 data to model their kinematic properties and infer the galaxy's rotational dynamics and tidal interactions.

Contribution

It provides the first detailed kinematic model of SMC star clusters' rotation disk, linking their motions to tidal effects from the LMC.

Findings

SMC clusters form a tilted, edge-on rotation disk.

Velocity dispersion perpendicular to the disk is twice that in the plane.

Tidal interactions with LMC influence the SMC's kinematic behavior.

Abstract

We report results of 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 are 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-node, proper motion in right ascension and declination and systemic velocity, rotation velocity and velocity dispersion. The SMC cluster rotation disk seems to be at some level kinematically synchronized with the rotation of 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 twice as big 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 would not seem sufficient to explain the observed kinematic behaviors in the SMC.