Annu. Rev. Astron. Astrophys. 1981. 19: 373-410 Copyright © 1981 by Annual Reviews. All rights reserved

1. INTRODUCTION

Extragalactic radio sources are conveniently divided into three distinct regions: 1. the diffuse clouds of emission which may exist up to megaparsecs away from the parent galaxy or quasar, 2. the narrow jets and hot spots which are thought to represent the transport of energy between the nuclei and the diffuse clouds, and 3. the compact sources which are coincident with galactic nuclei and quasars.

Regions 1 and 2 have been the subject of several recent reviews (e.g. Miley 1980, Fomalont 1981) and are not discussed further here. In this article we consider primarily the compact extragalactic radio sources, which are characterized by small angular sizes (<< 1"), high surface brightness with peak brightness temperatures ~ 10¹² K, variability on characteristic time scales of months to years, and flat or inverted radio spectra.

It is widely supposed that the compact radio sources reflect the "central engine" which produces the large amount of energy required by the extended emission regions. Studies of their spectra, time variations, structures, and particularly their kinematics are expected to give some insight into the manner in which relativistic electrons gain and lose energy and how they are transported out of the nucleus, and in this way perhaps lead to a better understanding of the nature of the fundamental energy source. Because of their small size it is relatively straightforward to determine the position of the compact radio sources with an accuracy better than one arcsecond. This is usually sufficient to permit an unambiguous optical identification, particularly since the optical counterparts are often prominent in that wavelength band (Condon & Jauncey 1974, Weiler & Johnston 1980).

Nearly all BL Lac objects, most quasars, and many radio galaxies contain compact radio nuclei. Compact radio sources are also found in other active galaxies, as well as in apparently normal spirals and ellipticals. However, the properties of the compact radio sources do not appear to be related to the type of optical identification, other than that the BL Lac objects appear to show the extremes of inverted spectra, high polarization, and variability (Condon 1978).

The measured angular sizes of compact radio sources range upward from a few tenths of a milliarcsec. There is no clearly defined upper limit to what is called a compact source, but a useful operational definition of a compact source is one that shows self absorption above the lowest frequencies normally used in radio astronomy, say ~ 100 MHz. This leads to a maximum angular dimension ~ 0.1 arcsecond (see Equation (1). below). Corresponding linear dimensions range from ~ 0.01 parsec in nearby galaxies, to several hundred parsecs for some of the larger, more distant quasars. Assuming isotropic emission, the total radio luminosity may range from ~ 10³⁷ to 10⁴⁶ erg s^-1, but the volume emissivity is typically of the order of 1 milliwatt km^-3 for a wide range of luminosities and linear dimensions.

It is interesting to note that there are many compact Galactic sources, including the Galactic center itself, with comparable surface brightness and volume emissivity, but with luminosities of only ~ 10³³ erg s^-1 and dimensions of only a few AU (see, for example, Kellermann et al. 1977a, Lo et al. 1977, Geldzahler et al. 1979). The relation between the compact Galactic and extragalactic sources is not clear, although the Galactic center source may represent a link between the more powerful extragalactic nuclei, and other compact Galactic sources.

In Section 2 we review the basic theoretical framework used to interpret compact radio sources, and in the following sections we discuss their spectra, their time variations, their brightness distributions, and their variations. In Section 7 the space distribution and evolution of the compact radio sources are compared with that of the extended radio sources, and the apparent differences are discussed in the light of their physical properties.

Other recent reviews that discuss the nature and properties of compact radio sources include those by Condon (1978), O'Dell (1978), Wardle (1978), Kellermann (1980a), and Pauliny-Toth (1981). Relevant reviews of the infrared and optical radiation (Stein et al. 1976, Rieke & Lebofsky 1979, Angel & Stockman 1980) and in the X-ray (Bradt 1980) band have also been published. The material included in this review covers the literature up to September 1980.