Substructure in the Globular Cluster System of the Milky Way: The Highest Metallicity Clusters

Abstract

An analysis of the kinematical and spatial properties of the highest metallicity globular clusters in the Galaxy, having metallicities of [Fe/H] > -0.8, indicates that these objects do not comprise a homogeneous population. Three subsystems are identified among these clusters. 1. The highest mass clusters with log (M/M☉) > 5.5 exhibit a very slow net rotation with a speed of vrot = 24 ± 23 km s-1 and vrot/σlos = 0.3, which is indicative of a centrally condensed, relatively high metallicity subsystem. 2. Roughly half of the lower mass clusters appear to be located in an elongated barlike structure that passes through the Galactic center and that has properties similar to those of the central stellar bar of the Milky Way. 3. The remaining lower mass clusters exhibit very rapid net rotation, with a rotation speed of vrot = 164 ± 6 km s-1 and vrot/σlos = 6. These clusters are located in the Galactic plane, within a ring of 4-6 kpc radial distance from the Galactic center. The highest mass clusters may have formed during relatively advanced stages of the dissipative evolution of the inner Galactic halo. Although the lower mass bar clusters have kinematical properties that are similar to the highest mass clusters, their spatial distribution suggests that they may be associated with the formation of the Galactic stellar bar or bulge. The lower mass ring clusters appear to be real disk objects. They may represent a stage in cluster formation that was intermediate between that of the halo globular clusters and the oldest extant open clusters.