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

5.2 Compact Sources

The compact radio sources are usually associated with quasars or BL Lac type objects. Others, however, such as 3C 84, 3C 111, and 3C 120, are found in the nuclei of active galaxies and even in some normal spiral or elliptical galaxies (Kellermann et al. 1976, Condon & Dressel 1978, Crane 1979, Shaffer & Marscher 1979, Jauncey et al. 1981, van Breugel et al. 1981). When mapped with VLB interferometer systems, the smallest scale structure typically shows a complex brightness distribution, which can be described by an asymmetric "core-jet" structure with a bright flat spectrum component at the end of a low surface brightness "jet" (Readhead et al. 1979, Readhead 1980, Pauliny-Toth et al. 1981). On a somewhat larger scale (e.g. 3C 395 and CTD 93), the sources may appear remarkably symmetric, with a ratio of component separation to component size of up to 30 to 1 (Phillips & Mutel 1980).

In the core-jet sources, the core components appear to be self-absorbed at long wavelengths and so are relatively more prominent at the shorter wavelengths (Readhead 1980, Pauliny-Toth et al. 1981). Within the core the smallest components are strongest at the short wavelengths (Kellermann 1978, Bååth et al. 1981). Typically, the peak component brightness temperature observed is ~ 10^12-12 K as expected from a compact synchrotron source with inverse Compton cooling (Kellermann & Pauliny-Toth 1969).

Because the surface brightness of the jet components is low compared with that of the core, it is difficult to map their structure, although at least in the case of 3C 345, high resolution observations show that the jet appears to break up into a number of relatively bright regions (Bååth et al. 1981). At the longer wavelengths, where the jet components are stronger, the resolution has generally been inadequate to map the jet structure in detail.

By combining observations made at different wavelengths, it is sometimes possible to trace the structure of the jet from a milliarcsecond out to tens of milliarcseconds. For four or five of the brightest sources, where observations exist over a wide range of wavelength, the jet is seen to curve through an angle 20 to 40 degrees as the distance to the core decreases (Readhead et al. 1978b, Readhead et al. 1979, Pauliny-Toth et al. 1981). High resolution maps of two sources that have been studied with VLBI systems are shown in Figure 5.

Figure 5. (a) Contour maps of the source CTD 93 (18 cm) taken from Phillips & Mutel (1980). (b) Contour map of 3C 273 at 6cm taken from Pauliny-Toth et al. (1981).

Recent observations of a number of stronger compact sources, made with arcsecond resolution, show that they often possess a diffuse structure extending seconds of arc away along an angle which is continuous with that of the compact structure but which is a factor of a thousand or more larger (Davis et al. 1978, Perley & Johnston 1979, Perley et al. 1980). Observations of even lower resolution made with high dynamic range indicate that there may be very low surface brightness features which are aligned with the compact structure, but which extend up to several megaparsecs away (Reich et al. 1980). But it has not been clearly established whether these weak features are associated with the compact sources.