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12.3.1. Types of Identifications

Identifications of extra-galactic radio sources are with a variety of objects, for example, as defined by Matthews, Morgan, and Schmidt (1964), with

  1. normal spiral galaxies
  2. Seyfert galaxies
  3. E, or elliptical, galaxies which are often the brightest member of a cluster

  4. D galaxies, which are similar to elliptical galaxies, but contain an extended halo

  5. dB or Dumbell galaxies, which contain a double nucleus imbedded in a common halo

  6. N galaxies, which contain a compact bright nucleus with a strong emission-line spectrum, superimposed on a faint nebulous envelope

  7. QSO's or quasi-stellar objects (quasars).

The above sequence is very roughly in increasing order of absolute radio luminosity, although there is a wide spread of luminosity within each class. In general the spiral galaxies are relatively weak radio emitters, while the various types of ellipticals (E, D, dB, N) are considerably more intense. Some of the giant ellipticals, however, contain a very weak radio source coincident with its nucleus (Heeschen, 1970).

In recent years there has been a growing realization that the distinction between the various optical categories is largely subjective and may vary depending on the observer, the size of the telescope, and the distance of the object. For example, the prototype quasar, 3C 48, is now sometimes classed as an N galaxy (e.g., Morgan, 1972). In another case the optical jets from the quasar 3C 273 and the giant elliptical galaxy M87 show surprising similarity.

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