The Seyfert galaxies constitute the most common type of galaxies that host an active galactic nucleus (AGN), i.e. a nucleus containing ionized gas where the energy source is not related to O and B stars. The class is subdivided into the two types
Intermediate subtypes between these two classes are used when the permitted lines show both broad and narrow components (see Osterbrock 1989 Chapter 11.3).
The broad permitted emission lines are supposed to emerge from a central `broad line region' of high density. The physical state of the broad line region is poorly understood. It is still not known whether it consists of one or several distinct components, whether its overall structure is spherical or flat, and whether the dominating motions are rotational, possibly a rapidly rotating accretion disk, or radial. It is still debated whether the gas is photoionized or collisionally heated. (For a recent discussion of these problems see Dumont et al. 1998). The broad line region is supposed to be surrounded by a torus of absorbing material which permits the nuclear continuum and broad emission lines to be seen only from certain directions. The narrow permitted line components and the forbidden lines come from a more extended `narrow line region' outside this torus (Fig. 17). Thus, the appearance, and thereby the classification, may depend also on the orientation of the object with respect to the observer according to the so called `unified scheme' (Antonucci & Miller 1985; Antonucci 1993). Recent observations suggest that low luminosity AGNs (LLAGNs) with emission line and X-ray luminosities low compared with typical Seyfert galaxies reside in a considerable number of galaxies (Ho et al. 1997).
In many cases narrow jet-like radio structures, consisting of an electron-positron plasma (Wardle et al. 1998) and presumably produced by interaction between an accretion disk and a central black hole, emerge from the nucleus out to large distances. They are generally assumed to be aligned with the rotation axis of the accretion disk. Other forms of circumnuclear activity include so called `outflow cones' of high excitation gas, in general most easily observed in the light of the [O III] 4959, 5007 emission lines. More recent observations suggest that low luminosity AGNs (LLAGNs) with emission line and X-ray luminosities low compared with typical Seyfert galaxies reside in a considerable number of galaxies (Ho et al. 1997).
For a review of the AGN phenomenon see e.g. Blandford et al. (1990).