Classification of active galaxies and the prospect of a unified phenomenology

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The observational classification of active galaxies is reviewed and its likely meaning discussed. There is only one kind of active galactic nucleus (AGN). The observed variety arises from three degrees of freedom: (1) Dust opacity, which produces the distinction between Type 1 and Type 2 AGN. (2) Viewing angle of a relativistic jet, which produces the distinction between blazars and Type 1 AGN. (3) Duty cycle of activity (i.e., fraction of time spent "on") which produces the distinction between radio-loud and radio-quiet AGN. The duty cycle may be related to the mass of the spheroidal component of the parent galaxy. A fourth degree of freedom is, of course, the overall luminosity. For theories of AGN one needs a clear qualitative phenomenology and one needs to formulate a quantitative phenomenology.
Original languageEnglish
Pages (from-to)309-334
JournalPublications of the Astronomical Society of the Pacific
Publication statusPublished - 1 May 1987


  • Active Galaxies
  • Astrophysics
  • Classifying
  • Galactic Nuclei
  • Phenomenology
  • Active Galactic Nuclei
  • Bl Lacertae Objects
  • Cosmic Dust
  • Mass Distribution
  • Quasars
  • Radio Galaxies
  • Seyfert Galaxies


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