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 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, globular clusters. The current sample of confirmed globular clusters in NGC 5128 is now 605 with 564 of these having radial velocity measurements, the second largest kinematic database for any galaxy. We have performed a new kinematic analysis of the globular cluster 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 globular clusters as functions of galactocentric distance and metallicity. Our results indicate that the metalpoor globular clusters have a very mild rotation signature of 26 ± 15 km s −1 . The metal-rich globular clusters 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 globular clusters with ages less than 5 Gyr in the literature appear to have different kinematic properties than the old, metal-rich globular cluster subpopulation. The mass and mass-to-light ratios have also been estimated using the globular clusters as tracer particles for NGC 5128. Out to a distance of 20 ′ , we have obtained a mass of (5.9 ± 2.0) × 10 11 M ⊙ and a mass-to-light ratio in the B-band of 16 M ⊙ /L B⊙ . Combined with previous work on the ages and metallicities of its globular clusters, 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.

An imaging and spectroscopic study of the planetary nebulae in NGC 5128 (Centaurus A)

“…The measured line splitting is 280 km s −1 (Table 12), and both components are narrow, with the redder component the stronger one. The redder component is within 40 km s −1 of the radial velocity of the GC (649 km s −1 , Woodley et al 2010a), and this was confirmed in the FLAMES spectra by measuring the velocity of the GC Hβ absorption line (see Sect. 5.3).…”

“…The radial velocity of this GC was measured by Woodley et al (2007) as 689 ± 16 km s −1 and so was taken as the template velocity. Cross correlating the other GC spectra against this template, and comparing the measured velocities with those from Woodley et al (2007Woodley et al ( , 2010a for 17 GCs enabled a modified template velocity to be determined which better matched, on average, the velocities from the literature, and resulted also in consistent velocity for HGHH-C4 within the error. All cross-correlation measurements were preformed using the IRAF fxcor task.…”

“…In order to determine if the PN and the GC are related, the most direct method is to compare their radial velocities. The velocities of some of these GCs are available in the literature (Peng et al 2004a;Woodley et al 2007Woodley et al , 2010a. Velocities can also be determined by cross-correlation using the GIRAFFE spectra.…”

How old are the stars in the halo of NGC 5128 (Centaurus A)?

“…The amount of HI associated with the central dust lane is 3.5 × 10 8 M , and there is about the same amount of H 2 (Charmandaris et al 2000). At < ∼ 10 9 M (Charmandaris et al 2000), the total gas mass is much less than the total dynamically determined mass of the galaxy of (1.3 ± 0.5) × 10 12 M within a galactocentric radius of 45 kpc (Woodley 2006;Woodley et al 2007Woodley et al , 2010a, equivalent to a total stellar mass ∼10 11 M .…”

Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter

“…Globular clusters (Puzia et al 2004;Bergond et al 2006;Romanowsky et al 2009;Shen & Gebhardt 2010;Schuberth 2010;Woodley et al 2010) or planetary nebulae (PNe, see, e.g. Napolitano et al 2001Napolitano et al , 2002Mendez et al 2001;Peng et al 2004;Douglas et al 2007;Coccato et al 2009;Teodorescu et al 2010;Napolitano et al 2011) have made it possible to extend kinematics up to 5−7 R eff , from the ≈R eff achievable with only stellar observations.…”