radio galaxy

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radio galaxy

An extragalactic radio source identified with an optical galaxy whose radio-power output lies in the range 1035 to 1038 watts. These sources often show symmetric tripolar structure (see radio-source structure) and may be distinguished from normal galaxies, such as M31, whose radio emissions roughly follow the optical contours of the galaxy and whose radio powers are much lower, in the range 1030 to 1032 watts. Powerful radio sources are always located in elliptical galaxies, many of which are the central dominant member of a cluster of galaxies. Radio galaxies with diameters greater than one megaparsec are now known. An example of such a giant radio galaxy is 3C 236, which is a double radio source whose radio emission spans almost six megaparsecs of space, making it one of the biggest radio galaxies in the Universe.

Radio galaxies are a form of active galaxy. High-luminosity radio galaxies sometimes share some characteristics of quasars, such as broad optical emission lines and a bright optical nucleus in the galaxy and are termed broad-line radio galaxies (BLRG).

The similarities, in addition to their shared radio-source structure, have led several astronomers to propose that the radio quasars and the very powerful FR II radio galaxies are the same type of object viewed at different inclinations to the line of sight. These unification scenarios suggest that when radio galaxies are observed along the radio source axis, the continuum emission from the central black hole is relativistically beamed into the line of sight, brightening the galaxy nucleus to appear as a quasar. When the system is viewed at a larger inclination, the continuum emission is no longer beamed into the line of sight and only the ‘host’ galaxy is seen. An optically thick torus around the nucleus may contribute to hiding it from sight. In this scenario, a BLRG is seen at some intermediate angle that is near the division of quasars and radio galaxies. Comparisons of orientation-independent properties of the systems, such as extended optical line emission, extended radio flux, the size and magnitude of the host galaxy, and cluster membership, broadly support the unification schemes. Similar schemes exist to unite the two types of Seyfert galaxy, and to identify BL Lac objects as the beamed apparition of the less powerful FR I radio galaxies.

High-redshift radio galaxies, such as those selected from the 3C catalog, are exceptionally luminous systems in both optical and radio emission. The galaxies possess optical emission-line nebulae and an optical-ultraviolet continuum (in the rest frame of the galaxy) that are extended over tens to hundreds of kiloparsecs. These structures are elongated and aligned with the direction of the radio-source axis, in an alignment effect, thought in part to be due to star formation induced in the surrounding medium by the passage of the radio jets. Other distant radio sources, known as compact steep-spectrum radio sources, show very steep radio spectra and subtend only a very small angular scale in the radio waveband. These are thought to be powerful radio galaxies in the early stages of their formation, when the radio jets are prevented by expanding from the active nucleus by a dense environment.

Radio Galaxy


a galaxy characterized by unusually high radio-frequency radiation in comparison with normal galaxies, such as the Milky Way or Andromeda galaxies. Radio galaxies are the most numerous extragalactic radio sources; they resemble, on the one hand, quasars, and, on the other hand, normal (spiral) galaxies in terms of the nature of their radio-frequency radiation. However, it has not yet been determined (1975) whether radio galaxies constitute a special group of objects or merely a particular stage in the evolution of any galaxy. The vast majority of radio galaxies are giant elliptical galaxies, which include galaxies with peculiarities in their nuclei, such as Seyfert and N galaxies. A red shift has been measured for approximately 100 radio galaxies, making it possible to determine their distances from the earth. The most distant such radio source is the radio galaxy 3C295, which has a red shift of 0.46. Radio galaxies have a “luminosity” in the radio-frequency range of 1045–1045 ergs/sec (as opposed to 1037–1038 ergs/sec for normal galaxies).

Regions that emit radio radiation usually have a fairly complex structure and are characterized by extended (transparent) and compact (opaque) regions. Most radio galaxies consist of two radio sources located at appreciable distances from the galaxy’s optical component. The radio-emitting region often contains several components of smaller size. Radio-frequency radiation from radio galaxies is often linearly polarized, which indicates homogeneity of the magnetic field on a large scale. Many objects are characterized by a variability in the radio-frequency radiation; this is chiefly the case with compact regions. In certain radio galaxies this variability is accompanied by variability of brightness in the optical band as well.

Radio-frequency radiation from radio galaxies appears to be synchrotron radiation; that is, it results from the motion of ul-trarelativistic electrons—electrons traveling at speeds close to the speed of light—in weak magnetic fields. According to the observed flux of radio-frequency radiation the energy alloted to the relativistic particles is extraordinarily high—approximately 1052 ergs in compact sources and 1057–1061 ergs in extended sources. The latter value is approximately 10–4 of the total energy of the galaxy. The nature of the intensity and polarization variability with wavelength and time indicates that dense clouds of relativistic particles are periodically ejected; the clouds subsequently expand and become transparent. The power of such explosions is of the order of 1052 ergs. An extended source requires approximately one explosion each year over a period of approximately 108 years (approximately 1048ergs are released when an ordinary supernova explodes).

The most difficult problems associated with radio galaxies are the determination of their evolution, the nature of the energy sources, and the way in which the energy is imparted to relativistic particles. No satisfactory hypothesis has yet been advanced to explain the phenomenon of radio galaxies.


Pacholczyk, A. G. Radioastrofizika. Moscow, 1973. (Translated from English.)
Zel’dovich, la. B., and I. D. Novikov. Reliativistskaia astrofizika. Moscow, 1967.


radio galaxy

[′rād·ē·ō ′gal·ik·sē]
A galaxy that is emitting much energy in radio frequencies often from regions devoid of visible matter.
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