|Annu. Rev. Astron. Astrophys. 1981. 19:
Copyright © 1981 by . All rights reserved
5.1 Extended Sources
A substantial fraction of extended radio sources contains a compact core (see Schilizzi 1976, Preuss et al. 1977, Gopal-Krishna et al. 1980). In such cases a clear optical identification is generally possible, since the core position, unlike that of the complex extended emission, can be uniquely defined. In the typical radio galaxy, where the symmetric extended emission dominates, the compact component, if present, is found coincident with the galactic nucleus to within the measurement accuracy of ~ ± 1 arcsec.
In most cases the flux density of the compact nucleus is rather weak, and it is difficult to map their structure with current techniques. With the exception of Cygnus A. these sources are not typical in that a significant fraction of their flux density is in the compact core. More typically, less than 10 percent of the flux density at cm wavelengths is in the compact core, and only Cygnus A is sufficiently strong to allow a detailed study.
As illustrated in Figure 4, the compact nuclei generally show the same asymmetric "core-jet" type of structure that is observed for the isolated sources, although the associated extended emission is usually svmmetrically placed with respect to the compact structure and associated galaxy or quasar.
The nuclear components are usually aligned roughly along a direction close to that joining the extended components which may be up to hundreds of megaparsecs away, although sometimes there is significant curvature (Pauliny-Toth et al. 1976a, Kellermann 1978, Readhead et al. 1978a, Fomalont 1981, Schilizzi et al. 1979, Preuss et al. 1980, Kellermann et al. 1981, Linfield 1981, Kus et al. 1981). This alignment implies a preferred axis in each source which extends from a few light years or less to far out into intergalactic space, and which must last over time scales of at least 108 years.
Figure 4. (a) Composite map of the radio galaxy NGC 1275 reconstructed from data given by Miley & Perola (1975), Romney (1979), and Preuss et al. (1979). The shaded region shows schematically an H II cloud which partially obscures the northeast part of the nucleus.
The compact nuclei of extended radio sources have spectral properties comparable to those of the isolated comp1act sources, and often there is strong IR or X-ray emission associated with the radio emission. Although there are no published results of long term monitoring of these sources, in general they do not appear to show the same extensive flux density variations of the type typically observed in the isolated compact sources (R. Ekers, private communication). In some radio galaxies, such as 3C 111 and 3C 390.3, the nuclei do show pronounced flux density variations (Hine & Scheuer 1980) but, as discussed above, in these objects the compact nucleus contains a significant fraction of the total flux density.
Very compact components have never been found in the lobes of symmetric extended sources, although Kapahi & Schilizzi (1979) have found "hot-spots" in the lobes of extended quasars, with linear sizes ~ 1 kpc.