The Flattening of Globular Clusters

TL;DR

This study investigates the flattening of globular clusters across different galaxies, finding that flattening correlates with host galaxy luminosity but not with cluster luminosity or horizontal branch morphology.

Contribution

It provides new evidence that globular cluster flattening depends on host galaxy properties and refutes previous claims linking flattening to horizontal branch types.

Findings

Brightest clusters are rounder than faint ones in nearby galaxies.

Cluster flattening is independent of luminosity within the LMC.

No correlation between flattening and horizontal branch morphology.

Abstract

In the three nearest luminous galaxies, the Milky Way System, the Andromeda Galaxy and NGC 5128 the brightest globular clusters are rounder than the faintest ones. On the other hand (contrary to some previous results) the flattening of individual LMC clusters is found to be independent of their luminosities. This suggests the possibility that the relationship between the flattening and luminosity of clusters might depend on host galaxy luminosity. No significant differences are found between the intrinsic flattening distributions of Galactic old halo, Galactic young halo and Galactic bulge/disk clusters. Such a dependence might perhaps have been expected if tidal forces (which are largest at small Galactocentric distances) removed angular momentum from globular clusters. The preliminary conclusion by Norris that clusters with blue horizontal branches are more flattened than red HB clusters is not confirmed by the larger data base that is now available. In other words there is no evidence for the puzzling claimed correlation between the flattening and the horizontal branch morphology of Galactic globular clusters.