We have used ACS and WFC3 cameras on board HST to resolve stars in the halo of NGC 5128 out to 140 kpc (25 effective radii, Reff) along the major axis and 70 kpc (13 Reff) along the minor axis. This dataset provides an unprecedented radial coverage of stellar halo properties in any galaxy. Color-magnitude diagrams clearly reveal the presence of the red giant branch stars belonging to the halo of NGC 5128 even in the most distant fields. The V-I colors of the red giants enable us to measure the metallicity distribution in each field and so map the metallicity gradient over the sampled area. The stellar metallicity follows a shallow gradient and even out at 140 kpc (25 Reff) its median value does not go below [M/H]~−1 dex. We observe significant field-to-field metallicity and stellar density variations. The star counts are higher along the major axis when compared to minor axis field located 90 kpc from the galaxy centre, indicating flattening in the outer halo. These observational results provide new important constraints for the assembly history of the halo and the formation of this gE galaxy.

This paper reviews what has been learned about the old stellar population of NGC 5128, the only large elliptical galaxy close enough that we can currently observe individual stars as faint as the horizontal branch. Although its galaxy type is still a matter of debate, the uncertainties over distance are now largely resolved; comparison of five stellar distance indicators gives d= 3.8±0.1 Mpc. The globular cluster system, which was once perplexingly invisible, is now known to be predominantly old with a substantial metal-rich component. The globular cluster system (GCS) total population and luminosity function are normal and the clusters follow the same fundamental plane relation as those in the Milky Way and M31. Finally, the halo out to at least ∼7reff is dominated by metal-rich stars which are also predominantly old, with age and metallicity tantalizingly similar to the majority of globular clusters.

We review our recent studies of the globular cluster system of NGC 5128. First, we have obtained low-resolution, high signal-to-noise spectroscopy of 72 globular clusters using Gemini-S/GMOS to obtain the ages, metallicities, and the level of alpha enrichment of the metal-poor and metal-rich globular cluster subpopulations. Second, we have explored the rotational signature and velocity dispersion of the galaxy's halo using over 560 globular clusters with radial velocity measurements. We have also compared the dependence of these properties on galactocentric distance and globular cluster age and metallicity. Using globular clusters as tracer objects, we have analyzed the mass, and mass-to-light ratio of NGC 5128. Last, we have measured the structural parameters, such as half-light radii, of over 570 globular clusters from a superb 1.2-square-degree Magellan/IMACS image. We will present the findings of these studies and discuss the connection to the formation and evolution of NGC 5128.

At less than 4 Mpc distance the radio galaxy NGC 5128 (Centaurus A) is the prime example to study the supermassive black hole and its influence on the environment in great detail. To model and understand the feeding and feedback mechanisms one needs an accurate determination of the mass of the supermassive black hole. The aim of this review is to give an overview of the recent studies that have been dedicated to measure the black hole mass in Centaurus A from both gas and stellar kinematics. It shows how the advancement in observing techniques and instrumentation drive the field of black hole mass measurements and concludes that adaptive optics assisted integral field spectroscopy is the key to identify the effects of the AGN on the surrounding ionised gas. Using data from SINFONI at the ESO Very Large Telescope, the best-fit black hole mass is MBH = 4.5(+1.7, −1.0) × 107 M⊙ (from H2 kinematics) and MBH = (5.5 ± 3.0) × 107 M⊙ (from stellar kinematics). This is one of the cleanest gas-versus-star comparisons of a MBH determination, and brings Centaurus A into agreement with the MBH−σ relation.

In this paper we review the various high precision methods that are now available to determine the distance to NGC 5128. These methods include: Cepheids, TRGB (tip of the red giant branch), PNLF (planetary nebula luminosity function), SBF (surface brightness fluctuations), and Long Period Variable (LPV) Mira stars. From an evaluation of these methods and their uncertainties, we derive a best-estimate distance of 3.8±0.1 Mpc to NGC 5128 and find that this mean is now well supported by the current data. We also discuss the role of NGC 5128 more generally for the extragalactic distance scale as a testbed for the most direct possible comparison among these key methods.

TeV gamma rays have been observed from blazars as well as from radio galaxies like M 87 and Centaurus A. In leptonic models, gamma rays above the pair production threshold can escape from the ultrarelativistic jet, because large Lorentz factors reduce the background photon densities compared to those required for isotropic emission. Here we discuss an alternative scenario, where very high energy photons are generated as secondaries from ultrahigh energy cosmic rays interactions in the cores of active galactic nuclei. We show that TeV gamma-rays can escape from the core despite large infrared and ultraviolet backgrounds. For the special case of Centaurus A, we study whether the various existing observations from the far infrared to the ultrahigh energy range can be reconciled within this picture.

NGC 5128 at 3.8 Mpc is the nearest large elliptical galaxy and is ideally suited to a detailed study of its planetary nebula population. Two spectroscopic programmes are summarised. More than 1200 PNe candidates are known from imaging campaigns in NGC 5128 and accurate radial velocities of 1070 have been measured with the VLT FLAMES/Giraffe spectrometer. From these data a variety of studies of the galaxy kinematics are enabled, such as search for PN sub-groups, representing the relics of accretion of small galaxies. Emission line spectra were observed with VLT FORS and the light element abundances determined for 40 PNe through photoionization modelling. A spread in O abundance of about 0.9 dex is found but no obvious radial gradient out to 19 kpc. Comparison of the O abundance from these PN with the metallicity for the stellar population in the neighbourhood of the PN will probe the star formation and enrichment history of the galaxy. Full results from this analysis will be presented in a forthcoming paper.

We present an analysis of the radial velocities and velocity dispersions for 27 bright globular clusters in the nearby elliptical galaxy NGC 5128 (Centaurus A). For 22 clusters we combine our new velocity dispersion measurements with the information on the structural parameters, either from the literature when available or from our own data, in order to derive the cluster masses and mass-to-light (M/L) ratios. The masses range from 1.2 × 105M⊙, typical of Galactic globular clusters, to 1.4 × 107M⊙, similar to more massive dwarf globular transition objects (DGTOs) or ultra compact dwarfs (UCDs) and to nuclei of nucleated dE galaxies. The average M/LV is 3±1, larger than the average M/LV of globular clusters in the Local Group galaxies. The correlations of structural parameters, velocity dispersion, masses and M/LV for the bright globular clusters extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical galaxy nuclei and DGTOs/UCDs. The detection of the mass-radius and the mass-M/LV relations for the globular clusters with masses greater than ~ 2 × 106M⊙ provides the link between “normal” old globular clusters, young massive clusters, and evolved massive objects.

We present preliminary results from a new wide-field photometric investigation of the globular cluster system of NGC 5128. NGC 5128 is the nearest giant elliptical and thus its globular cluster system serves as prototype for those of this important galaxy type. We have obtained images from IMACS at the Magellan telescope in excellent seeing that allow us to not only resolve globular clusters but derive their structural parameters. This extensive database provides a wealth of data to study the globular clusters in unprecedented detail. We discuss here the resolution of the clusters and their preliminary structural parameters.

To extend our recent study of structural parameters of Globular Clusters (GCs) in the giant elliptical NGC 5128 (Gómez et al. 2006), we have obtained new high resolution images with the Magellan 6.5m at Las Campanas Observatory and the IMACS camera. Our images cover an area of 1.2 × 1.2 sq. degrees and have a seeing between 0.4 and 0.7'', thus making it possible to resolve a significant fraction of the GC population, which is now virtually entirely observed given the large spatial and magnitude range.

Convolution techniques allow us to find and to study the cluster candidates using the stellar PSF and analytical profiles for their light distribution. We therefore are able to derive structural parameters like size (effective and core radii), ellipticity, position angle and concentration. This is combined with the extensive database in the Washington photometric system by Harris et al. (2004) to select clusters according to shape, extendedness and Washington magnitudes and colours, which has proven very useful in discriminating GCs from foreground stars and background galaxies and for deriving metallicities.

Our first results for the central field (15'×15’) indicate that the GCs populate a large range of (MV, rh) parameter space, with typical sizes of Galactic GCs as well as much larger. Combined with our previous data, we find a continuum of clusters in this diagram, with no clear distinction between a variety of different ‘types’ often referred to, such as UCDs, Faint Fuzzies, etc.

In addition, a large fraction of GCs show a significant ellipticity whereas a paucity of round clusters is observed. We also confirm with a large (~250) sample that metal-rich clusters are smaller than metal-poor ones, which is in keeping with the general trend found in other galaxies. (e.g. Jordán et al. 2005)