Extragalactic radio sources are objects that emit a continuum of radio wavelengths and that lie beyond the confines of the Galaxy were divided in the 1950s into two classes depending on whether they present spatially extended or essentially "starlike" images. Radio galaxies belong to the former class, and quasars (short for "quasi-stellar radio sources") to the latter. The distinction is somewhat arbitrary, because the ability to distinguish spatial features in cosmic radio sources has improved steadily and dramatically over the years, owing to Sir Martin Ryle's introduction of arrays of telescopes, which use aperture-synthesis techniques to enhance the angular resolution of a single telescope. Apart from the smaller angular extent that arises from being at a greater distance, many objects originally classified as quasars are now known to have radio structures that make them indistinguishable from radio galaxies. Not every quasar, however, is a radio galaxy. For every radio-loud quasar, there exist 20 objects having the same optical appearance but not the radio emission. These radio-quiet objects are called QSOs for quasi-stellar objects. Henceforth, the term quasars will be used to refer to both quasars and QSOs when the matter of radio emission is not under discussion.

The most powerful extragalactic sources of radio waves are double-lobed sources (or "dumbbells") in which two large regions of radio emission are situated in a line on diametrically opposite sides of an optical galaxy. The parent galaxy is usually a giant elliptical, sometimes with evidence of recent interaction. The classic example is Cygnus A, the strongest radio source in the direction of the constellation Cygnus. Cygnus A was once thought to be two galaxies of comparable size in collision, but more recent ideas suggest that it is a giant elliptical whose body is bifurcated by a dust lane from a spiral galaxy that it recently swallowed. The collisional hypothesis in its original form was abandoned because of the enormous energies found to be needed to explain the radio emission.

The radio waves coming from double-lobed sources are undoubtedly synchrotron radiation, produced when relativistic electrons (those traveling at nearly the speed of light) emit a quasi-continuous spectrum as they gyrate wildly in magnetic fields. The typical spectrum of the observed radio waves decreases as a power of increasing frequency, which is conventionally interpreted, by analogy with the situation known to hold for the Galaxy in terms of radiation by cosmic-ray electrons, with a decreasing power-law distribution of energies. The radio waves typically also show high degrees of linear polarization, another characteristic of synchrotron radiation in well-ordered magnetic fields.

A given amount of received synchrotron radiation can be explained in principle by a variety of assumed conditions. For example, a high energy content in particles (relativistic electrons) combined with a low content in magnetic fields will give the same radio luminosity as a low energy content in particles combined with a high content in magnetic fields. The American astrophysicist Geoffrey Burbidge showed that a minimum value for the sum results if one assumes that the energy contents of particles and fields are comparable. The minimum total energy computed in this way for Cygnus A (whose distance could be estimated from the optical properties of the parent galaxy) proved to be between 10⁶⁰ and 10⁶¹ ergs.

A clue to the nature of the underlying source of power came from aperture-synthesis studies of the fine structure of double-lobed radio galaxies. It was found that many such sources possess radio jets that point from the nuclei of the parent galaxies to the radio lobes. It is now believed, largely because of the work of Sir Martin Rees and Roger Blandford, that the nucleus of an active galaxy supplies the basic energy that powers the radio emission, the energy being transported to the two lobes by twin beams of relativistic particles. Support for this theoretical picture exists, for example, in VLA maps (those made by the Very Large Array of radio telescopes near Socorro, N.M., U.S.) of Cygnus A that show two jets emerging from the nucleus of the central galaxy and impacting the lobes at "hot spots" of enhanced emission (Figure 2). Other examples of this type are known, as are "head-tail" sources such as NGC 1265 where the motion of an active galaxy through the hot gas that exists in a cluster of galaxies has apparently swept back the jets and lobes in a characteristic U shape.

Excerpt from the Encyclopedia Britannica without permission.