THE KINEMATICS OF THE GLOBULAR CLUSTER SYSTEM OF NGC 5128 WITH A NEW LARGE SAMPLE OF RADIAL VELOCITY MEASUREMENTS

Abstract

New radial velocity measurements for previously known and newly confirmed globular clusters (GCs) in the nearby massive galaxy NGC 5128 are presented. We have obtained spectroscopy from LDSS-2/Magellan, VIMOS/Very Large Telescope, and HYDRA/Cerro Tololo Inter-American Observatory from which we have measured the radial velocities of 218 known, and identified 155 new, GCs. The current sample of confirmed GCs in NGC 5128 is now 605 with 563 of these having radial velocity measurements, the second largest kinematic database for any galaxy. We have performed a new kinematic analysis of the GC system that extends out to 45' in galactocentric radius. We have examined the systemic velocity, projected rotation amplitude and axis, and the projected velocity dispersion of the GCs as functions of galactocentric distance and metallicity. Our results indicate that the metal-poor GCs have a very mild rotation signature of 26 ± 15 km s−1. The metal-rich GCs are rotating with a higher, though still small signature of 43 ± 15 km s−1 around the isophotal major axis of NGC 5128 within 15'. Their velocity dispersions are consistent within the uncertainties and the profiles appear flat or declining within 20'. We note the small sample of metal-rich GCs with ages less than 5 Gyr in the literature appear to have different kinematic properties than the old, metal-rich GC subpopulation. The mass and mass-to-light ratios have also been estimated using the GCs as tracer particles for NGC 5128. Out to a distance of 20', we have obtained a mass of (5.9 ± 2.0) × 1011 M☉ and a mass-to-light ratio in the B band of 16 M☉/LB☉. Combined with the previous work on the ages and metallicities of its GCs, as well as properties of its stellar halo, our findings suggest NGC 5128 formed via hierarchical merging over other methods of formation, such as major merging at late times.