Active galaxy nucleus

Active Galactic Nuclei(AGN) are particularly bright galactic nuclei that must therefore be the site of very high energy production processes in the form of radiation at various wavelengths.

The NGC 1365 galaxy seen in the visible region by the VLT, and in the X-ray region by Chandra, shows an AGN; click to enlarge (Credit: ESO/VLT and NASA/CXC).

AGNs are generally classified into three types:

• radio galaxies;
• Seyfert galaxies;
• quasars.

In fact they are all the same object: i.e. a central super-massive black hole of several millions to several billion solar masses, accreting matter and observed from different angles from the Earth.

Radio galaxies

These are ordinary-looking galaxies, usually giant elliptical galaxies or lenticular galaxies, but which powerfully emit in the radio region. The radiation emitted can be hundreds of times more powerful than that of so-called normal galaxies, but an example of a source called Cygnus A is known which is a million times brighter than our galaxy. An important characteristic of radio galaxies is that there are two lobes, sometimes at several thousand light years from the central black hole, in which most of the radio emission is concentrated. These are the ends of jets of matter expelled at high speed by the black hole.

Seyfert galaxies

These are most often spiral galaxies and were observed for the first time in 1943 by Karl Seyfert. Examples are the NGC 1410 galaxies in the constellation of Eridan and M77 in the constellation of the Whale. They are brighter than the average galaxy not only in the radio region but also in the visible.

Quasars

Discovered in the early 1960s, are very distant objects, particularly active and numerous more than a billion years ago. The central black hole of the galaxies was copiously supplied in gas because of the collisions between galaxies which were more frequent than today. One particular class of quasar is blazars, a typical example being the BL Lacertae AGN. In the unified AGN model, this is a jet of matter from a quasar directed towards us.

The unified AGN model; depending on the angle of observation, different AGNs can be seen
Credit: Prof. H. E. (Gene) Smith