THE KINEMATICS OF THE GLOBULAR CLUSTER SYSTEM OF NGC 5128 WITH A NEW LARGE SAMPLE OF RADIAL VELOCITY MEASUREMENTS
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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/VLT, and Hydra/CTIO 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 564
of these having radial velocity measurements. We have performed a new kinematic
analysis of the GC system that extends out to 45 arcmin 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 pm 15) km/s. The
metal-rich GCs are rotating with a higher, though still small signature of (43
pm 15) km/s around the isophotal major axis of NGC 5128 within 15 arcmin. Their
velocity dispersions are consistent within the uncertainties and the profiles
appear flat or declining within 20 arcmin. 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 arcmin, we have obtained a mass
of (5.9 pm 2.0) x 10^(11) M_(sun) and a mass-to-light ratio in the B-band of 16
M_(sun)/L_(B,sun). Combined with 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.
