Radio Galaxy

radio galaxy [′rād·ē·ō ′gal·ik·sē]

(astrophysics)

A galaxy that is emitting much energy in radio frequencies often from regions devoid of visible matter.

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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.

REFERENCES

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

I. V. GOSACHINSKII