Space velocities of globular clusters. III. Cluster orbits and halo substructure

We have compiled a catalog of absolute proper motions of globular clusters from various sources. The sample consists of 38 clusters, from which most o f the southern ones (15 clusters) were measured in our previous papers in t his series. We have integrated orbits assuming two different Galactic poten tial models adopted from the literature and have calculated orbital paramet ers. The uncertainties associated with the orbital parameters were derived in a Monte Carlo approach, and we conclude that, overall, at the present le vel of measurement errors, orbital differences due to Galactic potential mo dels are not significant. Three metal-poor clusters are found to have orbits similar to prototypical metal-rich disk clusters. These clusters are NGC 6254 (M10), NGC 6626 (M28) , and NGC 6752. We interpret this as a potentially significant constraint o n the formation of the disk. It is thus possible that part of the inner met al poor halo is the low-metallicity tail of the thick disk. In this case, t he ages of these clusters indicate that the formation of the disk partially overlapped with that of the halo. The clusters classified as "young halo" or "red horizontal-branch" by Zinn show a radially anisotropic velocity dis tribution, their orbits are of high total energy, with apocentric radii lar ger than 10 kpc and highly eccentric. In this sense they may represent an a ccreted component of our Galaxy. We also discuss omega Cen's orbit characteristics in the view of an accrete d origin. We investigate the effect of the orbital motion on the internal d ynamics of clusters. Adopting the formalism from Gnedin & Ostriker and thei r destruction rates due to two-body relaxation, we find that, in most cases , this internal process is more important than the destruction processes du e to disk and bulge shocking. Hubble Space Telescope (HST) observations arg ue that NGC 6397's luminosity function is depleted at the faint end, and th is is blamed on its high total destruction rate. We propose a list of clust ers with similar destruction rates that may also have depleted luminosity f unctions. We also note the bias toward deriving higher destruction rates in studies that statistically assign tangential velocities based on a kinemat ic model of the globular cluster system, in contrast to the rates derived f rom the measured tangential velocities. Clusters prone to such biases are t hose that have circular orbits (kinematically thick-disk clusters) and some of those with orbits of high total energy.