25/08/2026
An Active Galactic Nucleus (AGN) is a compact, extremely energetic region at the centre of a galaxy, emitting radiation across the electromagnetic spectrum (i.e., from radio to gamma rays). Thus, a galaxy hosting an AGN is classified as an active galaxy.
The observed emission does not have stellar origin. It is believed that this phenomenon arises from the accretion of matter by a supermassive black hole at the centre of its host galaxy. The properties and brightness of an AGN depend on the mass of the central black hole, the rate of gas accretion into the black hole, the orientation of the accretion disk, the presence or absence of jets, and the amount of dust around the nucleus.
Numerous subclasses of AGN have been defined. However, the primary distinction of AGN relates to the presence of a jet. Thus, an AGN with a powerful relativistic plasma jet driven by its central black hole is classified as radio-loud. An AGN without a jet is classified as radio-quiet. The latter includes the vast majority of AGNs (90%+).
AGNs are further classified as quasars, blazars, Seyferts, radio galaxies, and LINERs. Quasars are extremely distant and luminous nuclei that outshine their host galaxy. Blazars are variable AGNs with relativistic jets pointing towards Earth. Seyfert galaxies are nearby spiral galaxies with bright nuclei that exhibit characteristic emission line spectra. Radio galaxies are giant elliptical galaxies with massive radio lobes and jets viewed from the side. Finally, LINERs (Low-Ionisation Nuclear Emission-line Regions) are galaxies with weak nuclear emission lines.
Centaurus A (also known as NGC 5128, Caldwell 77, Arp 153, amongst many others) is a galaxy located at a distance of nearly 12.5 million light-years in the constellation of Centaurus. It was discovered on the 29th of April 1826 by James Dunlop.
Centaurus A is the closest radio galaxy and the closest blazar. Hence, it has been studied extensively. Centaurus A hosts a supermassive black hole with a mass of 55 million solar masses. Monitoring programmes revealed that the inner parts of the jet are moving at about half the speed of light. The jet and the associated lobes of Centaurus A emit across the electromagnetic spectrum. The most dominant component is observed at radio wavelengths. Moreover, there is an infrared component due to the thermal glow of dust heated by the jet's passage. Most of the optical component is obscured by dust. However, faint features have been detected by optical telescopes. Observations with the Chandra X-ray Observatory revealed the X-ray component of the jet, extending for thousands of light-years.
Image: Composite image of Centaurus A. It was created using X-ray data from the Chandra X-ray Observatory (blue), optical data from ESO's 2.2 metre MPG/ESO telescope (blue, green, red), and millimetre data from ESO's APEX (orange).
Image Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)
