Active Galactic Nuclei (AGN)

Galaxies are now known to contain Supermassive Black Holes in their centers. In most galaxies (perhaps 90 percent) the central black hole has very little observable effect on the galaxy. In the remaining about 10 percent, a supply of gas becomes available, perhaps in Interacting Galaxies and Mergers. When the gas sinks to within a distance of a few light years, the black hole pulls gas into an accretion disk roughly the size of our solar system. The gas becomes heated to high temperatures as it falls into the accretion disk.

The accretion disk can be very luminous and in the brightest sources (called quasars) can out shine all the stars in the galaxy. The objects in which the black hole is accreting gas in this way are said to have Active Galactic Nuclei (AGN). In addition to the photons from the accretion disk, AGN may also generate winds or outflows. In fact outflows seem to be a general phenomenon associated with disks around compact objects, especially young stellar objects. In about 10 percent of AGN called Radio Galaxies, almost exclusively in large elliptical galaxies, the outflows emit synchrotron emission at radio wavelengths.

Most if not all galaxies may go though an Active Galactic Nuclei (AGN) phase at some point. The life time of the AGN phase is likely to be in the range 10 to 100 million years.

The supply of gas which feeds the accretion disk can have two additional effects. Dense gas and dust around the accretion disk causes our view of the AGN to be blocked in about 2/3 of the objects. In these cases, we do not see the light from the accretion disk directly. The light which is absorbed by the dust is typically re-radiated in the infrared part of the spectrum. In addition, the gas can also form stars, some times quite rapidly, producing a “starburst”.

Active Galactic Nuclei are often found in Starburst Galaxies.