The structure of Small Magellanic Cloud star clusters

Abstract

The variations in the structural parameters and the internal dynamical evolution of star clusters in the Milky Way and the Large Magellanic Cloud (LMC) caused by the different gravitational field strengths that they experience has been revealed thanks to recent observational data sets. We report here possible indications of such differential tidal effects in the structural parameters of star clusters in the Small Magellanic Cloud (SMC), which is nearly ten times less massive than the LMC. A key contribution to this study is the consideration of the SMC as a triaxial spheroid; from this, we estimate the de-projected distances to the SMC center of the statistically significant sample of analyzed star clusters. By adopting a 3D geometry of the SMC, we avoid the spurious effects caused by assuming that a star cluster observed along the line-of-sight is close to the galactic center. When inspecting the relationships between the de-projected distances and the star cluster sizes (represented by the 90% light radii), their eccentricities, masses, and ages, we find: (i) The star cluster sizes are not visibly affected by tidal effects, because relatively small and large objects are spread throughout the SMC body; (ii) star clusters with large eccentricities (≥0.4) are preferentially found at de-projected distances smaller than ∼7−8 kpc, although many star clusters with smaller eccentricities are also found occupying a similar volume; and (iii) star clusters more massive than log(M/M⊙) ∼ 4.0 are among the oldest star clusters, and they are generally found in the outermost SMC region with a relatively low level of flattening. These findings contrast with those of the more elongated, generally younger, less massive, and innermost star clusters.