Radio Galaxies and Quasars
Galactic and Extra-Galactic Radio Astronomy
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
... 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.