Recent improvements in image formation techniques using interferometer baselines of thousands of kilometers (VLBI) now permit images of compact radio sources to be made with resolutions better than one milliarcsecond. Detailed radio pictures of quasars and galactic nuclei are now possible on a scale which is typically of the order of a few parsecs even for the most distant objects. For nearby galaxies, it is considerably less.
When mapped in detail (see Figure 13.4), the compact sources show a variety of structural forms. The great majority have asymmetric structure containing a bright region plus an elongated feature which resembles the jets seen on larger scales. These jetlike features often break up into a number of distinct components with different surface brightness and self-absorption cutoff frequency.
Equation (13.19) indicates that for a wide range of magnetic field strength, there is a characteristic size of compact radio sources which varies with the wavelength of observation. Although there may be a big spread of the opacity in each source, individual components are most readily observed at the wavelength where the flux density is near a maximum (opacity of the order of unity). Thus, when observed over a range of frequency, individual sources show structural features ranging from a few tenths of a milliarcsecond or less at short centimeter wavelengths to a few hundredths of an arcsecond or more at longer wavelengths. The integrated spectrum, which is the sum of many peaked self-absorbed components, often is remarkably flat and shows an average spectral index near zero. Comparison of component sizes and self-absorption cutoff frequency, c, indicate magnetic field strengths in the range of 10-4 to 10-2 gauss (Equation 13.12), but the observational uncertainties are very large as the derived value of B depends on c5 and 2.
When there is an extended jet (see Section 13.4.1), it always lies on the same side of the core as the compact jet. Characteristically, the compact features are curved through an angle of a few tens of degrees, although in some cases the curvature extends through more than ninety degrees. The curvature is most pronounced near the inner region of the jet as it emerges from the core. The outer parts of the compact jets are usually aligned with the larger-scale jets, which are up to hundreds of kiloparsecs away. The alignment of these features over size scales ranging up to a factor of 100,000 means that the large jets are focused and collimated within a region less than a parsec across. This remarkable feature of extragalactic radio sources implies a unique axis which extends from a parsec to a few hundred kiloparsecs, and a current activity with a "memory" extending back at least 105 to 106 years.
Some sources have a well-defined self-absorption cutoff frequency, usually at a relatively low frequency of a few hundred MHz. Above this frequency, the spectra are characteristic of transparent sources. VLBI observations often show that these sources have complex angular structure with overall angular sizes about 0.1 arcsecond, corresponding to the relatively low self-absorption cutoff frequency. Other sources of this type which are generally referred to as Steep-Spectrum Compact Sources have two similar well-separated components with no evidence for any jet structure.
There appears to be no obvious difference in structure between the compact components in sources with weak extended structure and the weak compact sources which are located near the center of strong double-component extended sources.