Extragalactic radio sources

More than 10 000 individual discrete sources have been detected by radio surveys made at frequencies from 10 MHz to 5 GHz. Although only a few hundred of these are reliably identified with galaxies or quasars, it is widely believed that the overwhelming majority are extragalactic. As a result of the rapid improvement in the sensitivity and the resolution of radio telescopes, the coordinates of many of the cataloged sources are being routinely measured with an accuracy of about 1" and with special effort to a few tenths of a second of arc. Their angular structure is being mapped with a resolution of a few arc seconds by means of sophisticated multielement interferometer arrays; and for the very bright sources, tape recording interferometers with continental and intercontinental baselines give a resolution three or four orders of magnitude better. The form of the radio frequency spectra is known in some detail between 38 MHz and 10 GHz for hundreds of sources, and for a smaller number from 10 MHz to 100 GHz. Yet, in spite of the many exciting new observational results, or perhaps because of them, many more problems in understanding the extragalactic sources have arisen than have been answered by the more recent observations.