Active Galactic Nuclei

Active galactic nuclei (AGNs) are perhaps the most violently energetic objects in the universe. AGNs are located at the centers of some galaxies—perhaps most galaxies—and emit a tremendous amount of energy, sometimes on the order of trillion times the output of the Sun. An AGN may outshine all the stars in its galaxy by a factor of 100. The energy of a typical AGN is generated in a volume smaller in diameter than our solar system, leading astronomers to conclude that AGNs are probably powered by supermassive black holes, that is, black holes containing a million to a billion or more times the mass of the Sun. The event horizon of such a black hole would be a small object by astronomical standards, with a diameter equal to perhaps one-twentieth of the distance from Earth to the Sun. Gas attracted by the black hole's gravity spirals inward, forming a rotating "accretion disk" (so-called because as matter approaches the black hole it accretes, or adds to, this disk). As the accretion disk spirals toward the event horizon of the black hole it is accelerated, compressed, and heated, causing much of its gravitational potential energy to be released in the form of radiation. This converted gravitational potential energy is the source of the tremendous outpourings of radiation that are observed from AGNs.

Much of the energy from AGNs is emitted as radio waves rather than as visible light. These waves are emitted by electrons moving in a helical path in a strong magnetic field at speeds near the speed of light. This is known as synchrotron radiation (after the machine called a synchrotron, a type of cyclotron that confines high-speed charged particles by using a magnetic field to force them to move in curved paths). AGNs also emit visible light, x rays, and gamma rays.

About 10% of AGNs have mirror-image jets of material streaming out from the nucleus in opposite directions and at right angles to the accretion disk, moving at nearly the speed of light. Near their sources, these jets tend to vary in brightness on rapid cycles of days to months in length. Such rapid variations indicate that the energy-producing nucleus is small, ranging in size from a few light days to a few light months in diameter. Size can be deduced from the time-scale of brightness variations. Coordinated changes across a jet's source imply that some sort of coherent physical process is affecting the jet from one side of its aperture to another at least as rapidly as the observed variation, and this cannot happen faster than the speed of light. Thus a brightness change of, say, one day implies a source no more than one light-day in diameter. (A light-day is the distance traveled by light in one day, 16 billion mi. [2.54 × 10¹⁰ km].)

There are several varieties of active galactic nuclei, including compact radio galaxies, Seyfert galaxies, BL Lacertae objects, and quasars. Compact radio galaxies appear as giant elliptical galaxies. Radio telescopes, however, reveal a very energetic compact nucleus at the center, which is the source of most of the energy emitted by the galaxy. Perhaps the best-known compact radio galaxy is M87. Recent observations provide strong evidence that this core contains a supermassive black hole. Seyfert galaxies look like spiral galaxies with a hyperactive nucleus; that is, a set of normal-looking spiral arms surround an abnormally bright nucleus. BL Lacertae objects look like stars, but in reality are most likely very active galactic nuclei. BL Lacertae objects exhibit unusual behaviors, including extremely rapid and erratic variations in observed properties, and their exact nature is not known. Quasars also look like stars, but they are now known to be simply the most distant and energetic type of active galactic nuclei. They may be more active than nearer AGNs because they are observed in a younger condition (i.e., being distant, their light has taken longer to reach us), and so are seen in a particularly vigorous stage of accretion.

AGNs may merely be unusually active galactic nuclei, as evidence is accumulating that many or even most galaxies have black holes at their centers. If this is true, AGNs may simply have unusually active central black holes. Recent observations have confirmed that our own galaxy has a black hole at its core that is about three million times as massive as the Sun. It emits far less energy than an AGN; this may be simply because there is less matter falling into it. Black holes do not emit energy on their own, but become visible by squeezing energy out of the matter that they are swallowing. If the space in the near vicinity of a galaxy's central black hole is relatively free of matter, then the galaxy's core will be relatively quiet, that is, emit little energy; if a sufficient amount of matter is available for consumption by the black hole, then an AGN results.

AGNs are a particularly active area of astronomical research; about one fifth of all research astronomers are presently engaged in investigating AGNs.