Julian H. Krolik

Objects with a great variety of names - QSOs (or quasars), blazars, Seyfert galaxies, radio galaxies, and sometimes liners(low ionization nuclear emission line galaxies) - are all grouped into the category active galactic nuclei (AGN) because they share a basic set of common properties: very small spatial extent (on the galactic scale),luminosity comparable to or greater than that of an entire galaxy, and substantial power radiated in frequency bands where stars emit very little if at all. In addition to this set subscribed to by all AGN, many show evidence for bulk motion at relativistic speeds. Somewhere inside each object there must be a system responsible for the tremendous amounts of energy released; because they share so many basic characteristics, it is generally thought that in each of the different varieties of active galaxy this "central engine" is built according to basic design that is common to all. The "specifications"for this central engine are exactly this list of common properties, and we begin by briefly elaborating on them. At present, observations only give upper limits on the sizes of these objects. Atmospheric "seeing" limits angular resolution of ground-based telescopes to -1 arcsecond, corresponding to -100 parsec(pc) in even the nearest AGN. Some AGN are strongly variable; in these, causality causality limits the size to the distance light can travel in a characteristic variability time. This limit is often considerably less than 1 parsec, but systematic studies of AGN variability are still in their infancy. Active galactic nuclei can be found over a very wide range of luminosity. The all-time record is ~ 1048 erg s-1, or more than 104 times brighter than an average galaxy, but luminosities this large are quite rare. At redshifts around 2, AGN with luminosities ~ 1046 erg s-1 existed in ~ 1% of galaxies , whereas at the present epoch a few percent of all galaxies contain AGN with luminosities ~ 1044 erg s-1. It is possible tat somewhat weaker AGN are still more common.

Perhaps most remarkable of all, whereas stars emit nearly all of their power in a frequency band a mere factor of 3 wide, and the range of stellar temperatures broadens that range for a galaxy by no more than another factor of 3, most AGN produce roughly equal amounts of power per logarithmic frequency band all the way from the mid-infrared to hard x-rays - a span of 107 in frequency. The exceptions(radio galaxies) produce such a large ratio of very low frequency(radio) power to optical that they, too, could hardly be stars.

Whatever constitutes the central engine, it almost certainly must have a very large mass. Two arguments lead to this conclusion. First, because the force due to radiation pressure falls off with distance from the source in exactly the same inverse square fashion as gravity, there is a critical luminosity to mass ratio beyond which a self-gravitating and radiating structure cannot exist. This is called the Eddington luminosity, and is appeq 4 × 104 in units of solar luminosities per solar mass. From this argument we infer that the central engines of active galaxies must have a mass at least ~ 106(L / 1044 erg s-1) Msun.

Second, the total active lifetime of an AGN must be at least ~ 108 yr. This is the minimum mean active lifetime derived from the observed frequency of AGN if all galaxies are occasionally active. It is possible that only a few percent of all galaxies have ever been active, but in that case the observed frequency of AGN means that they must have been active throughout the lifetime of the universe, ~ 1-2 × 1010 yr. Thus the minimum total energy released by an average AGN is ~ 1060 erg. It is possible to estimate the minimum accumulated mass in the central engine by supposing that this energy was derived from processing some sort of "Fuel. "Chemical fuels release ~ 10-9 of their rest-mass energy when burnt; nuclear reactions release ~ 10-3. Only the conversion of gravitational potential energy into heat when matter falls into a relativistically deep potential well produces energy with efficiency approaching unity in rest-mass units: Accretion onto a neutron star releases a fraction appeq 0.1-0. 2, accretion onto a maximally rotating black hole can release up to appeq 0.29. Even with these high efficiencies, the minimum accumulated mass for a typical AGN is still ~ 107 Msun, and if only a small fraction of galaxies ever become active, the minimum accumulated mass could be much greater than this.

It is the difficulty in understanding how such large masses are brought so close to the centers of galaxies that has led most astronomers to believe that the basic power source for an AGN is accretion into a relativistically deep potential, probably a massive black hole. Although a dense cluster of neutron stars cannot absolutely be ruled out, it seems less likely: If typical AGN are less than few light-days across(as the variability would in some cases suggest), the cluster would have to be so dense that stellar collisions would cause collapse to a black hole in less than the minimum active lifetime of ~ 108 yr.

The principal hurdle in bringing so much mass so close to the center of a galaxy is dumping the matter's angular momentum. Average stars in galaxies have 105 times more angular momentum than the maximum permitted for accretion onto a central black hole, and it is hard to identify mechanisms that efficiently remove angular momentum from material orbiting in galaxies. By contrast, energy can be lost comparatively easily by radiation. For this reason, it is generally thought that material approaches the central black whole along trajectories in which the energy is the minimum consistent with an orbit of that angular momentum. These trajectories taken together form a flat disk. At large distances from the center, it is possible that global disturbances in the gravitational field of the host galaxy remove angular momentum from the accreting matter; at small distances, friction between material on neighboring orbits may cause a slow outward transport of angular momentum and an associated slow sifting inward of the matter in the disk.

Energy can be released and transformed into radiation in a variety of ways. Within the disk, the same friction causing the angular momentum transport also causes local heating. The energy source, of course, is the slow fall of material in the gravitational field of the central object. This heat can then be lost by thermal radiation. Because the greatest amount of gravitational potential energy is the lost in the innermost rings of the disk, this inner region dominates the total power radiated, and its typical temperature (~ 105 K)forces most of the photons radiated by the disk to emerge in the ultraviolet.

Nonthermal mechanisms are possible also. Indeed, to explain the very broad range of photon energies seen, they are probably required. These generally involve populations of relativistic electrons(and sometimes positrons) in which the numbers of particles with a particular energy are proportional to a power of the energy. Relativistic particles are often distributed in energy in this way because the only characteristic energy scale relevant to them is the particle rest mass. Relativistic electrons can create new photons by the synchrotron mechanism and they can also multiply the energy of already-existing photons by large factors as a result of inverse compton scattering. Many suggestions have been made about how to produce such large quantities of very energetic electrons, but no consensus currently exists. Some, but not all, of these suggestions depend on accretion into a relativistic gravitational potential.

Electromagnetic effects are also likely to play a role in the energy release: The characteristic scale of magnetic field strength near the edge of the black hole is ~ 104(L / LE)(L / 1046 erg s-1)-1/2 G, though electric fields should (in most places) be efficiently shorted out by the high densities of ionized plasma. Magnetic fields threading the surface of the black hole and coupled to the external plasma can allow the rotational energy in the black hole to be tapped, and energy stored in the field itself can be released nonthermally if regions with oppositely directed field can be brought together to reconnect.

Our present understanding of the generation of bulk relativistic motions is even cruder. Possibilities include acceleration of plasma on magnetic field lines attached to a rotating black hole, hydrodynamic acceleration inside funnels formed by general relativistic dynamical effects along the rotation axis of a black hole, or acceleration by radiation pressure, but it is quite possible that the correct answer is something else altogether.

In sum, measured against the specifications for a central engine, massive black holes do better than any other model yet proposed. They are certainly sufficiently compact, having radii ~ 10-5(M / 108 Msun) pc; very large luminosities can be produced with a minimum mass in fuel consumed; the high energies afforded by the depth of their potential wells help in the creation of relativistic particles that can radiate over a broad range of frequencies; and they potentially provide sites for the bulk acceleration of matter to relativistic speeds. However, there are few statements that can be made on this subject with great confidence.

Additional Reading

Begelman, M. C. (1989). Physics of the central engine. In IAU Symposium 134: Active Galactic Nuclei, p. 141. Kluwer Academic publishers, Dordrecht.

Rees, M. J. (1984). Black hole models of active galactic nuclei. Ann. Rev. Astron. Ap. 22 471.

Rees, M. J. (1990). Black holes in galactic centers. Scientific American 263 (No. 5)56.

See also Active Galaxies and Quasistellar Objects, Jets; >Active Galaxies and Quasistellar Objects, X-Ray Emission; Cosmic Rays, Acceleration; Quasistellar Objects, Statistics and Distribution; Radiation, High-Energy Interaction with Matter; Radio Sources, Emission Mechanisms.

Adapted from The Astronomy and Astophysics Encyclopedia, ed. Stephen P. Maran