The discrete sources of radio emission were first distinguished from the general background radiation during the 1940's as a result of their rapid amplitude scintillations; and initially, it was thought that the scintillations were due to fluctuations in the intrinsic intensity of the discrete sources. Assuming that the dimensions of the sources could not greatly exceed the distance traveled by light during a typical fluctuation period of about 1 minute, it was concluded that the discrete sources were galactic stars located at relatively small distances from our solar system. Thus the term "radio star" was often used in referring to these sources.
The identification of two of the strongest sources, Virgo A and Centaurus A, with the nearby galaxies M87 and NGC 5128 by Bolton, Stanley, and Slee (1949) made it clear that at least some of the discrete sources were of extra-galactic origin. In 1954 an accurate position measurement of the strong source Cygnus A led to the identification of this source with a relatively faint 15th magnitude galaxy having a redshift z 0.06 (Baade and Minkowski, 1954), and the extragalactic nature of the discrete sources was generally recognized.
Although a few other radio sources were identified with galaxies during the 1950's, progress was slow because of the low accuracy of the radio source positions. By 1960, however, the positions of most of the strongest sources had been determined with an accuracy of about 10 seconds of arc and many were identified with various galaxy types. These galaxies, which are identified with strong radio sources, are generally referred to as "radio galaxies."
Most of the radio galaxies have bright emission lines, and so their redshift may be relatively easily determined. The faintest and most distant identified radio galaxy is the strong radio source 3C 295, which has a redshift of 0.46 and an apparent magnitude of about -21. This identification, which was made in 1960, was the result of accurate radio positions determined at Caltech and at Cambridge; it culminated a long search for distant galaxies and stimulated the search for galaxies of even higher redshift.
Continued efforts to identify distant galaxies were concentrated toward sources of small diameter and high surface brightness on the reasonable assumption that these were most easily observed at a large distance. A primary candidate was 3C 48, which had an angular size less than 1 second of arc, and was at the time the smallest strong source known. Accurate position measurements made in 1961 resulted in what appeared to be a unique identification with a 16th-magnitude stellar object having a faint red wisp extending away from it. The absence of any other optical visible object near the radio source and the later discovery of significant night-to-night variations in light intensity led to the reasonable conclusion that 3C 48, unlike other radio sources, was a true radio star. Soon two other relatively strong sources, 3C 286 and 3C 196, were also identified with "stars," and it appeared that more than 20% of all sources were of this class. The optical and radio properties were surprisingly dissimilar for the three objects, and there were no unique radio properties to separate them from radio galaxies.
Early efforts at interpreting the emission-line spectrum of 3C 48 were relatively unsuccessful, although the possibility of a large redshift was apparently considered. By 1962 most of the lines were thought to be identified with highly excited states of rare elements.
The identification of 3C 273 with a similar stellar object, however, again cast doubt on the galactic interpretation which by 1963 was widely accepted. 3C 273 was tentatively identified in Australia with a 13th-magnitude star from a moderately accurate position determined with the 210-foot telescope. The position and identification were confirmed by a series of lunar occultations. These showed that the radio source was double, one component being within 1 second of the optical image, and the other component being elongated and coincident with a jet-like extension to the star. The identification was beyond question, although one wonders why it had not been made much earlier, as 3C 273 was the brightest then unidentified source, of small angular size and located in an unconfused region of the sky near the galactic pole.
The optical spectrum of 3C 273 showed a series of bright emission lines which could be identified only with the Hydrogen Balmer series, but with a redshift of 0.16. This redshift was confirmed when the H line was found near the predicted wavelength of 7590 Å in the near infrared. Adopting this redshift of 0.16 then led to the identification of the MgII lines appearing at 3239 Å.
A re-inspection of the 3C48 spectrum gave a redshift of 0.37 if a strong feature at 3832 Å was identified with MgII. Other lines could then be identified with 0II, NeIII, and NeV. Additional spectra taken of other similar sources led to the identification of CIII (1909 Å), CIV (1550 Å) and finally Ly-, permitting redshifts as great as 3.5 to be measured.
Radio sources in this hitherto unrecognized class are usually referred to as quasi-stellar radio sources, QSS, or quasars. Follow ing the identification of the first quasars it was realized that they all had a strong UV excess, and the search for further quasar identifications was simplified by looking for a very blue object located at the position of radio sources. In fact, many such objects were found which optically appear to be quasars, but are radio-quiet. These were originally referred to as Blue Stellar Objects (BSO), Quasi-Stellar Galaxies (QSG), or Quasi-Stellar Objects (QSO). Today the word "quasar" is generally used to refer to the entire class of stellar-type objects with large apparent redshifts, while QSO and QSS refer to radio-quiet and radio-active quasars, respectively.
It is now widely accepted that the radio emission from both galaxies (including our own Galaxy) and quasars is due to synchro tron emission from relativistic particles moving in weak magnetic fields. The amount of energy required in the form of relativistic particles is, however, very great, and the source of energy and its conversion into relativistic particles has been one of the outstanding problems of modern astrophysics. The remainder of this chapter is devoted to a description of the observed properties of radio galaxies and quasars, and how these are interpreted in terms of the synchrotron mechanism.