The Kinematics and Dynamics of the Globular Clusters and Planetary Nebulae of NGC 5128

Abstract

A new kinematic and dynamic study of the halo of the giant elliptical galaxy NGC 5128 is presented. From a spectroscopically confirmed sample of 340 globular clusters and 780 planetary nebulae, the rotation amplitude, rotation axis, velocity dispersion, and total dynamical mass are determined for the halo of NGC 5128. The globular cluster kinematics were searched for both radial dependence and metallicity dependence by subdividing the globular cluster sample into 158 metal-rich ([Fe/H]> −1.0) and 178 metal-poor ([Fe/H]< −1.0) globular clusters. Our results show that the kinematics of the metal-rich and metal-poor subpopulations are quite similar: over a projected radius of 0 − 50 kpc, the mean rotation amplitudes are 47 ± 15 and 31 ± 14 km s −1 for the metal-rich and metalpoor populations, respectively. There is a indication within 0 − 5 kpc that the metal-poor clusters have a lower rotation signal than in the outer regions of the galaxy. The rotation axis shows an interesting twist at 5 kpc, agreeing with the zero-velocity curve presented by Peng and coworkers. Within 5 kpc, both metal-rich and metal-poor populations have a rotation axis nearly parallel to the north-south direction, which is 0 o , while beyond 5 kpc the rotation axis twists ∼ 180 o . The velocity dispersion displays a steady increase with galactocentric radius for both metallicity populations, with means of 111 ± 6 and 117 ± 6 km s −1 within a projected radius of 15 kpc for the metal-rich and metal-poor populations; however, the outermost regions suffer from low number statistics and spatial biases. The planetary nebula kinematics are slightly different. Out to a projected radius of 90 kpc from the center of NGC 5128, the planetary nebulae have a higher rotation amplitude of 76 ± 6 km s −1 , and a rotation axis of 170 ± 5 o east of north, with no significant radial deviation in either determined quantity. The velocity dispersion decreases with galactocentric distance. The total mass of NGC 5128 is found using the tracer mass estimator, described by Evans et al., to determine the mass supported by internal random motions and the spherical component of the Jeans equation to determine the mass supported by rotation. We find a total mass of 1.0 ± 0.2 × 10 12 M ⊙ from the planetary nebula data extending to a projected radius of 90 kpc. The similar kinematics of the metal-rich and metal-poor globular clusters allow us to combine the two subpopulations to determine an independent estimate of the total mass, giving 1.3 ± 0.5 × 10 12 M ⊙ out to a projected radius of 50 kpc. Lastly, we publish a new and homogeneous catalog of known globular clusters in NGC 5128. This catalog combines all previous definitive cluster identifications from radial velocity studies and HST imaging studies, as well as 80 new globular clusters with radial velocities from a study of M. A. Beasley et al. (in preparation).

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 .…”

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

“…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.…”

“…Given that no standard stars were taken for this purpose with the FLAMES in MEDUSA mode, a cross-correlation template was formed by averaging the three highest signal-to-noise GC spectra (HGHH-C6, HCH-21 and HGHH-C4), and shifting the other two spectra to that of HGHH-C4. 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.…”

“…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.…”

“…Currently there are 564 GCs with confirmed radial velocity measurements (van den Bergh et al 1981;Hesser et al 1984Hesser et al , 1986Harris et al 1992;Peng et al 2004a;Woodley et al 2005Woodley et al , 2010bRejkuba et al 2007;Beasley et al 2008). Woodley et al (2007) presented a combined kinematic study based on 780 PN velocities (Peng et al 2004c) and 340 GC velocities. There are some differences between the PN and GC kinematics such as higher rotation amplitude of 76 km s −1 for PN compared to 47 and 31 km s −1 for the metal-rich (red) and metal poor (blue) globulars, respectively.…”