How to classify AGNs
General and historical classification schemes
The classification of AGNs into subgroups is based on the history of research in this area. In particular, the discovery and general understanding of quasars, in the early 1960s, preceded the detailed study of the local members of this group, the Seyfert galaxies. This statement can be challenged by historians because of the seminal paper by Seyfert in 1943, in which he described the spectra of seven members of this group. However, the unusual features of these spectra were neglected for a long time, including the enormous velocities inferred from the widths of the emission lines and the extreme nuclear luminosities. These objects were rediscovered in the mid-1960s and studied in greater detail following the great interest in quasar spectra and redshifts.
The earlier detailed observations of local, low-redshift AGNs provided enough data to define several of the subgroups that are still used today: Seyfert 1 galaxies, Seyfert 2 galaxies, radio galaxies, quasars, blazars, LINERs, and so on. Additional observations of higher-luminosity, higher-redshift objects helped to refine the classification. Many observed properties of such sources have been described in Chapter 1.
Current AGN classification is based on higher-quality observations of a much larger number of sources, on better understanding of the physics of accretion and the line-emitting processes, and on the realization that many of the observed characteristics depend on the luminosity and inclination of the central source. The main subgroups of today are introduced in this book as type-I radio-quiet AGNs (in earlier years, Seyfert 1 galaxies and radio-quiet QSOs), type-I radio-loud AGNs (in earlier years, BLRGs, radio-loud QSOs, or QSROs), type-II radio-quiet AGNs (Seyfert 2s in earlier years), type-II radio-loud AGNs (in earlier years, NLRGs), LINERs, and blazars (BL-Lac objects and OVVs of earlier days). The following are three sets of somewhat different questions that can be posed to subclassify AGNs.