The Globular Cluster System of the Galaxy. IV. The Halo and Disk Subsystems

The spatial distributions, kinematics, and metallicities of 121 globular clusters are examined for evidence of distinct subpopulations in the cluster system. Clusters more metal-poor than (Fe/H) = -0.8 constitute the familiar halo population, while clusters more metal-rich than (Fe/H) = -0.8 have the properties of a disk system. There is a shallow but significant metallicity gradient with R in the halo population. The central concentrations and ellipticities of the halo and disk clusters are similar, while the latter are more evenly distributed in luminosity than the former. Excellent agreement is found between the properties of globular clusters and those of RR Lyrae variables and subdwarfs when these are also divided by metallicity at (Fe/H) = -0.8, which suggests that clusters are good tracers of the halo and old disk stellar populations. The scale height of the disk clusters appears to be about 500 pc, and it is argued that disk clusters are relics of a transient, thick-disk phase in the evolution of the Galaxy.