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

1. INTRODUCTION

The very story of the development of the social enterprises of science, technology, economy, and politics throughout the ages does in itself indicate something of the nature of the connections between them.

J.D. Bernal, Science in History

The largest, the furthest, the most powerful, and to some of us the most fascinating objects known in the Universe are to be found among the radio sources associated with some elliptical galaxies and QSOs. In recent years it has become apparent that they are also objects of considerable beauty.

This article deals with the appearances of strong extended extragalactic radio sources. Included as "extended" are sources whose sizes are comparable with or larger than a typical galactic diameter, and as "strong," those with intrinsic luminosities at 408 MHz greater than ~ 10²³ W Hz^-1. ⁽¹⁾ Compact radio sources have been reviewed recently by Kellermann (1978). Although there are several similarities between the radio properties of the nuclei of spirals and ellipticals (de Bruyn 1978), extended radio emission from spiral galaxies (van der Kruit & Allen 1976) will not be discussed here.

Previous reviews on the structure of extended extragalactic sources have been given by Moffet (1966), Miley (1976), and Willis (1978), Moffet (1975) and De Young (1976) have given more general reviews, the latter emphasizing the theoretical problems involved. Here we concentrate on the observations, stressing the results and literature of the past five years.

Three important types of inference can be attempted from radio source structure. First, the structures, combined with a modicum of imagination, provide clues to the processes that produce the radio sources. Second, with the addition of a large dollop of assumptions, the source structures give information about the physical parameters that characterize the environment of the sources. Third, stretching our credulity and fantasy to the full, source structure can be used to probe the geometry of the Universe. In this article we discuss only the first two. For an account of structure measurements applied to cosmology, see Miley (1976) and Ekers & Miley (1977).

Before considering the present state of our knowledge we review the changes that have taken place in our understanding of the structure of extended radio sources over the last three decades. The main factor in this evolution has been a steady progress in instrumentation and observational techniques. Therefore, although a detailed discussion of instrumental methods (e.g. Cohen 1969) is beyond the scope of this article, a brief summary of the main techniques used for measuring source structure is appropriate. The general properties of commonly observed source morphologies are considered in Section 2. Before dealing with the detailed characteristics of radio components (Section 4), we give an account in Section 3 of how some relevant physical parameters can be derived from the structures. Finally, the relationship of radio-source morphologies to the properties of the parent galaxies and clusters is reviewed in Section 5.

¹ We adopt a Hubble constant of H = 75 km/sec/Mpc throughout and have applied the relevant scaling to all parameters quoted. Back.