active galactic nucleus (AGN) The central region of a galaxy which emits far more radiation than can be produced by stars alone. A galaxy with such a nucleus is known as an
active galaxy. The first active galaxies were discovered in 1943 by C. K.
Seyfert, who noticed that some spirals have an exceptionally bright core; these are now termed
Seyfert galaxies.
Quasars, discovered in the 1960s, appear to be a more luminous version of Seyferts in which the nucleus is so bright that the surrounding galaxy can hardly be seen at all. About 10% of active galaxies have the additional characteristic of being strong radio sources, or ‘radio‐loud’. The radio emission from such
radio galaxies is often from regions well outside the galaxy itself known as
lobes, although in some cases there are radio
jets pointing back towards the centre of the galaxy, indicating that it is the nucleus which is the ultimate source of the energy. The widely accepted theory of the origin of the energy in active galaxies is that they contain a central black hole with a mass of up to 10
9 solar masses. Three main pieces of evidence lead to this conclusion. First, active galactic nuclei are often highly variable, meaning that the radiation is from a region not much bigger than the Solar System. Second, the spectral lines of an active galactic nucleus are often very broad, up to 10000 km/s, indicating the presence of something very massive in the centre. Third, there is no credible alternative. The radiation is produced not by the black hole itself but by gas in a surrounding
accretion disk. The viscosity in the disk causes gas gradually to spiral towards the black hole. As it does so, gravitational energy is converted into heat, and the hot gas then emits large amounts of radiation. Although this theory is widely accepted, there are still many unanswered questions, including how the jets seen in radio galaxies are produced. In addition to Seyfert galaxies, quasars, and radio galaxies, the range of active galaxies now includes
blazars and
BL Lac objects, plus many subspecies. However, it seems likely that at heart most active galactic nuclei are physically similar. Their diversity in outward appearance has two main causes. One is differences in the energy output of the ‘central engine’ itself—quasars, for example, are particularly powerful. The second factor is the angle from which the AGN is viewed. Surrounding the black hole and accretion disk is thought to be a ring of molecular gas and dust. Within the ring there are tiny dense clouds of gas which are heated by the radiation from the accretion disk. It is these clouds which are the source of the broad spectral lines seen from quasars and some types of Seyfert galaxy (Seyfert 1s). The clouds farther from the ring are also heated by the radiation from the accretion disk, and these clouds produce the narrow spectral lines seen from the other main class of Seyfert galaxies (Seyfert 2s). Radio jets (where they exist) are emitted along the axis of the accretion disk.
This model can explain many of the apparent differences between active galactic nuclei. If we view the AGN along the axis of the ring, we see the radiation emitted by the accretion disk and also broad spectral lines. However, if we view the AGN through the ring, this radiation and the broad spectral lines are hidden by the dust in the ring, and we can see only the narrow spectral lines produced by the clouds farther out. The appearance of a ‘radio‐loud’ AGN will also depend on the viewing angle. Viewed from close to the axis of the ring, a radio‐loud AGN may be classified as a blazar and the radio jet will often appear to be moving faster than the speed of light (
superluminal velocity); viewed from close to the plane of the ring, the AGN will be classified as a radio galaxy, with not many signs of activity in the nucleus, but with huge lobes of radio‐emitting plasma on either side of the galaxy.
It now seems quite likely that most galaxies have passed through an AGN phase. Astronomers have recently discovered that most nearby galaxies contain
supermassive black holes, including one of a few million solar masses in the centre of our Galaxy. Observations show that there were many more active galactic nuclei in the past than there are today, and it seems likely that the inactive nearby galaxies once contained an AGN, but for some reason in these galaxies the central engine has now run out of fuel. Possibly all the gas in the vicinity of the black hole has now been consumed.
