Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy

摘要

The High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescope has been used to measure the first radial velocities for stars belonging to 11 heavily reddened globular clusters in the direction of the inner Galaxy. The sample consists of the clusters Terzan 3, NGC 6256, IC 1257, NGC 6380 (=Ton 1), Ton 2 (=Pismis 26), Djorg 1, NGC 6540 (=Djorg 3), IC 1276 (=Pal 7), Terzan 12, NGC 6749, and Pal 10. Candidate cluster members were selected from a combination of previously published color-magnitude diagrams (CMDs) and new instrumental CMDs obtained with the Palomar 1.5 m telescope. The systemic velocities of Djorg 1 and Pal 10 should be considered provisional, since velocities are available for only two stars. For the remaining nine clusters, we have measured radial velocities for three to nine member stars. Using our HIRES spectra, we estimate metallicities of [Fe/H] -0.75 for both Terzan 3 and IC 1276, two clusters lacking previous metallicity estimates. The question of kinematic substructuring among the Galactic globular clusters is investigated using an updated catalog of globular cluster distances, metallicities, and velocities. It is found that the population of metal-rich globular clusters shows significant rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc of the Galactic center, the measured rotation velocity and line-of-sight velocity dispersion are similar to those of bulge field stars. We investigate claims that the metal-rich clusters are associated with the central Galactic bar by comparing the kinematics of the innermost clusters to that of the atomic hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich and metal-poor clusters bears a remarkable similarity to that of the gas, including the same noncircular motions that have traditionally been interpreted as evidence for a Galactic bar, or, alternatively, a nonaxisymmetric bulge. However, uncertainties in the existing three-dimensional Galactocentric positions for most of the clusters do not yet allow an unambiguous discrimination between the competing scenarios of membership in a rigidly rotating bar or in a bulge that is an oblate isotropic rotator. We conclude that the majority of metal-rich clusters within the central ~4 kpc of the Galaxy are probably associated with the bulge/bar, and not the thick disk.