Abstract / Description of output
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 language | English |
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Pages (from-to) | 309-334 |
Journal | Publications of the Astronomical Society of the Pacific |
Volume | 99 |
DOIs | |
Publication status | Published - 1 May 1987 |
Keywords / Materials (for Non-textual outputs)
- Active Galaxies
- Astrophysics
- Classifying
- Galactic Nuclei
- Phenomenology
- Active Galactic Nuclei
- Bl Lacertae Objects
- Cosmic Dust
- Mass Distribution
- Quasars
- Radio Galaxies
- Seyfert Galaxies