The taxonomy of AGNs tends to be rather confusing as we do not yet understand the physics underlying the AGN phenomenon. Undoubtedly some of the differences we see between various types of AGNs are due more to the way we observe them than to fundamental differences between the various types; this is a theme that will be revisited in Chapter 7. We will introduce the various types of AGNs that are generally recognized, and try to make clear as we proceed how these various types may or may not be related.

Seyfert Galaxies

Seyfert galaxies are lower-luminosity AGNs, with MB > -21.5 + 51og/h0 for the active nucleus the generally accepted criterion, due originally to Schmidt and Green (1983), for distinguishing Seyfert galaxies from quasars. A Seyfert galaxy has a quasar-like nucleus, but the host galaxy is clearly detectable. The original definition of the class (Seyfert 1943) was primarily morphological, i.e., these are galaxies with high surface brightness nuclei, and subsequent spectroscopy revealed unusual emission-line characteristics. Observed directly through a large telescope, a Seyfert galaxy looks like a normal distant spiral galaxy with a star superimposed on the center. The definition has evolved so that Seyfert galaxies are now identified spectroscopically by the presence of strong, high-ionization emission lines. Morphological studies indicate that most if not all Seyferts occur in spiral galaxies (Chapter 8).

Khachikian and Weedman (1974) were the first to realize that there are two distinct subclasses of Seyfert galaxies which are distinguished by the presence or absence of broad bases on the permitted emission lines.