A Discussion of Galaxies Indentified with Radio Sources.

A description is given of the optical forms of the identified extragalactic radio sources for which satisfactory plate material is available. These forms fall into two general categories: (a) "weak" sources, having luminosities ranging between t0 -t0 erg/sec radiated power; and (b) "strong" sources, having luminosities greater than t0 erg/sec. The spiral galaxies observed to be radio sources are in the "weak" source category. The most commonly encountered optical forms of the identified extragalactic radio sources are galaxies having elliptical-like inner regions surrounded by an extended envelope (the "D systems" of Morgan's classification); all of the D galaxies occur in the "strong" radio-source category. These D-galaxy radio sources frequently have double or multiple nuclei. A special case of the D galaxies with double nuclei is the "dumbbells," consisting of two separated, approximately equal, nuclei surrounded by a common envelope; the dumbbells also occur exclusively among the "strong" sources. The D radio galaxies occur over a range of 10000 to tin radio luminosity. A number of these galaxies occur in clusters of a special kind: clusters of galaxies which are dominated by a single D system, centrally located in its cluster, and very much larger and brighter than any other galaxy in the cluster. A number of other similar clusters of galaxies have been found to have a dominating D galaxy in which this galaxy is not a radio source. A list of some of the nearer clusters of this type is given. The radio sources tend to occur in clusters of richness 2 on Abell's system. The most luminous group of extragalactic radio sources is the so-called quasi-stellar sources, which have the optical appearance of accompanied by faint wisps; four of these objects are listed; they have a range in luminosity of the order of 10. Of somewhat similar optical appearance are the "N radio galaxies," which consist of brilliant, starlike nuclei located in relatively smaller and faint envelopes. The "N" radio galaxies are easily distinguished from the quasi-stellar sources spectroscopically, in the sense that the sp,,ectra of all four quasi-stellar sources have very broad emission lines, while the emission lines in the "N sources are much sharper. The "N" radio sources are in the strong radio- source group, but their mean radio luminosity is of the order of 50 times less than the quasi-stellar sources. The "N" radio sources tend to have a radio structure which is long and narrow with some small- scale components. The "dumbbell radio sources" have a marked tendency to have a simple radio structure comprised of a single, nearly symmetrical, Gaussian. The quasi-stellar radio sources have a variety of radio properties, all types of radio structure being present. There are faint optical extensions in NGC 5128, NGC 1316, and NGC 7720 which seem to be intimately related to the radio structure. In both Centaurus A and 3C 33 the radio sources are located near the axis of rotation of the galaxy. No highly flattened galaxies were observed among the "strong" sources; in this respect they differ sharply from the "weak" group, the majority of which are highly flattened spirals. The D galaxies are supergiant galaxies having a diameter along major axis which averages 50 kpc, whether they are radio sources or not. Thus, about 60 per cent of radio sources are smaller than, or comparable to, the size of the parent galaxy. The optical evidence is strongly against the earlier interpretation of radio sources in terms of a pair of colliding galaxies; there is not one source where this could be considered a more likely explanation than a single, peculiar galaxy.