|Annu. Rev. Astron. Astrophys. 1981. 19:
Copyright © 1981 by . All rights reserved
Recent improvements in image formation techniques using interferometer baselines of thousands of kilometers permit pictures of compact radio sources to be made with resolutions better than one milliarcsecond. It is now possible to obtain a detailed "radio" view of quasars and galactic nuclei on a scale of the order of a few light years, and in some cases considerably less. Only at radio wavelengths, where the effects of tropospheric fluctuations are relatively small, is it possible to even begin to approach, for nearby galaxies, scale lengths comparable to those of accretion disks which may exist around the compact massive objects thought to exist in quasars and galactic nuclei. Even the Space Telescope with its diffraction-limited operation will have a resolution two orders of magnitude poorer.
Very Long Baseline Interferometer (VLBI) systems are still relatively crude, compared with conventional interferometer arrays such as the Cambridge 5-km radio telescope, the Westerbork Synthesis Telescope, and particularly the newly completed VLA (Heeschen 1981). There are presently fewer antennas, they are not optimally located, and often have poor sensitivity and are difficult to calibrate at the shortest wavelengths, where the maximum resolution is achieved. The phase of the VLB interferometer response is generally lost, but images can be formed by means of several "self calibration" techniques which have recently been developed to deal with incomplete data (Readhead & Wilkinson 1978, Cotton 1979, Readhead et al. 1980, Schwab 1980, Cornwall & Wilkinson 1981).
Typical VLBI maps are made using four or five antennas, although up to eight have been successfully employed. Nevertheless, although there has been considerable improvement in recent years, the number of picture elements and the dynamic range are somewhat restricted in comparison with conventional aperture synthesis maps. Because of the instrumental limitation, and the dependence of the self-absorption cutoff frequency on angular size [Equation (1)], there is a characteristic measured size of compact radio sources which varies with the wavelength of observation according to Equation (5). Although there may be a wide range of opacity among sources, individual components are most readily observed at the wavelength where the flux density is near a maximum (opacity of the order of unity). If the opacity is greater (i.e. smaller dimensions), then the flux density is relatively small compared with other parts of the source. On the other hand, if the opacity is small, the component must be relatively large, and therefore substantially resolved by the interferometer. This conspires with the limited dynamic range to produce characteristic source sizes that are roughly proportional to the wavelength of observation. 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 (Pauliny-Toth et al. 1978b, Matveyenko et al. 1980, Bååth et al. 1981) to a few hundredths of an arcsecond or more at longer wavelengths (Wilkinson et al. 1979, Simon et al. 1980). In general, however, the dynamic range and the number of picture elements are at present insufficient to detect this broad range of surface brightness at any one wavelength.
When mapped in detail (see Figure 3), the compact sources show a variety of structural forms ranging from simple doubles to asymmetric sources containing a bright region plus an elongated low surface brightness component which may resemble the jets seen on larger scales (see, for example, Readhead et al. 1978b, Shaffer 1978, Kellermann 1978, Readhead 1980, Phillips & Mutel 1980). Generally there is also some largescale structure associated with the compact sources (e.g. Perley et al. 1980). Anywhere from a few percent to nearly all of the flux density may be in the compact components, and it is this fraction that determines the classification as a compact or extended source. Strictly speaking, however, there is no well-defined distinction between the two types of radio sources.