7.4 Models of active galaxies

• The standard model of an AGN consists of an accreting supermassive black hole (the engine) surrounded by a broad-line region contained within a torus of infrared emitting dust and a narrow-line region.

• Unified models attempt to explain the range of AGNs on the assumption that they differ only in luminosity and the angle at which they are viewed.

• One type of model attempts to unify radio-quiet AGNs. Type 1 Seyferts and type 2 Seyferts differ only in the angle at which they are viewed. Radio-quiet quasars (QSOs) are similar to Seyferts but much more powerful. Evidence for this model is strong.

• Another set of models, in which the engine emits a pair of jets, attempts to unify radio-loud AGNs. The observer sees a radio galaxy, a quasar or a blazar as the viewing angle moves from side-on to the jets to end-on. These models remain controversial and there is not yet a consensus on whether such a unification is possible.

• The difference between radio-loud and radio-quiet AGNs may lie in the angular momentum of their black holes. The faster-spinning holes may have arisen from mergers of black holes resulting from the collision of their host galaxies.