Published in Astronomy and Astrophysics Review 1998, Vol. 8, pp. 237-278.

For a postscript version of the article, click here.

Abstract. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be the prototype Fanaroff-Riley Class I "low-luminosity" radio galaxy, and as such it plays an important role in our understanding of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding unrivalled but still incomplete knowledge of its structure and dynamics.

The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past.

The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths, probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into the nucleus, presumably "feeding the monster". Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected to the circumnuclear disk, appears to precess on timescales of order a few times 10⁷ years.

This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying physical processes are outside its scope: they are briefly referred to, but not discussed.

Key words: Galaxies: active (11.01.2) - Galaxies: individual: NGC 5128 (11.09.1) - Galaxies: jets (11.10.1) - Infrared: galaxies (13.09.1) - Radio continuum: galaxies (13.18.1) - X-rays: galaxies (13.25.2)

Table of Contents

• INTRODUCTION
• Historical note
• Distance
• Systemic velocity
• Environment
• THE RADIO SOURCE AND ASSOCIATED FEATURES
• Overview
• The outer and middle lobes
• The inner lobes and outer jet
• Inner jet structure
• Optical jet features
• THE GALAXY
• Overview
• Globular clusters and metallicity
• Late-type objects
• Colours and reddening
• THE DUSTY DISK
• Overview
• Atomic and molecular gas
• Dust emission
• Polarization and extinction
• THE CENTRAL REGION
• Overview
• The circumnuclear disk
• Nuclear emission at infrared wavelengths
• The radio nucleus and nuclear jet
• X- and -ray emission from the nuclear region
• Variability of the nucleus
• KINEMATICS AND DYNAMICS
• Kinematical studies
• Dynamical models
• Evolutionary timescale
• THE NUCLEAR ABSORPTION SPECTRUM
• CONCLUDING SUMMARY
• REFERENCES