Annu. Rev. Astron. Astrophys. 1980. 18: 165-218 Copyright © 1980 by Annual Reviews. All rights reserved

1.3. Source Structure - The Story So Far

Almost thirty years ago Jennison & Das Gupta (1953) made the fundamental and exciting discovery that Cygnus A, the strongest observed extragalactic radio source, consists of two components which symmetrically straddle the associated optical galaxy and exceed it in size by an order of magnitude. It was gradually established during the next several years that double structure is a very common property of radio sources (Allen et al. 1963, Maltby & Moffet 1963) and that there are often bright regions of emission or "hot spots" within each of the two radio lobes (Allen et al. 1963).

These surprisingly accurate deductions about radio-source structures were made from an analysis of the interferometer fringe amplitudes. Since the mid-sixties earth-rotation aperture synthesis, including fringe-phase data, has been used to map the brightness distributions of several hundred radio sources. The maps confirmed the general features of radio sources that had been extracted from the fringe-amplitude data. Bright hot spots were found frequently at the edges of the intrinsically strongest double sources (e.g. Macdonald et al. 1968, Miley & Wade 1971, Fanaroff & Riley 1974).

Simple models were proposed to explain these observed features. One picture (Burbidge 1967) viewed the extended components as being composed of many thousands of condensed objects each having a mass of ~ 10³ M. An alternative model explained the structure of the radio components in terms of evolving plasmons produced in discrete outbursts from the parent nucleus, whose morphologies were governed by interaction with an intergalactic medium (e.g. van der Laan 1963, De Young & Axford 1967, Christiansen 1969, Mills & Sturrock 1970).

An intergalactic medium was also invoked to interpret the more complicated morphologies that were often revealed by detailed radio maps. In particular the tadpole-like "head-tail" radio sources associated with some galaxies in clusters (Ryle & Windram 1968, Hill & Longair 1971) were explained as double sources deformed by the motion of their parent galaxies through an intracluster medium (Miley et al. 1972).

Most of these measurements were made at frequencies of 1400 MHz and below. During the sixties advances in microwave electronics permitted interferometers to be operated at higher frequencies, resulting in the discovery that compact radio cores with relatively flat spectra are frequently embedded in the nuclei of all types of extended radio galaxies and quasars (Dent & Haddock 1965, Barber et al. 1966, Palmer et al. 1967, Mitton 1970). It became clear that the nuclei of these systems frequently remained active for a considerable fraction of the radio-source lifetimes.

These discoveries stimulated the development of models of radio sources in which the lobes are continuously powered by energy channeled or "beamed" quasi-continuously from their nuclei to hot spots in the lobes (Rees 1971, Scheuer 1974, Blandford & Rees 1974, Lovelace 1976, Blandford 1976, Benford 1979, Wiita 1978a, b).

During the past five years considerable progress has been made on the observational front. First there was the discovery that (at least for some nearby radio galaxies) narrow jets of radio emission emanate from the nuclear cores and extend far out into the radio lobes (Section 4.4). These jets constituted beautiful circumstantial evidence in favor of beam-type models and probably provide a new way of studying energy transport in radio sources. Second, the detection of optical emission has been reported both from some of these radio jets and from radio hot spots in the lobes (Section 4.2). Third, various alignments have been demonstrated both within the radio sources themselves and between radio and optical structures (Sections 4.3.3 and 5.2). These imply that the nuclear axis that defines the radio-source phenomenon often stays fixed to within a few degrees during the source lifetime and is fundamentally related to the axis of symmetry of the associated stellar system. Fourth, multifrequency comparisons of the total intensity and polarization distributions for several sources have yielded considerable information on the magnetic field distributions and on the physical conditions within the sources and in their environment (Sections 3 and 4).