Classifications of Active Galactic Nuclei According to the Traditional Unification Scheme

The figure illustrates the unification scheme of active galactic nuclei (AGN). The physical appearance of galactic nuclei is sketched in the parameter space spanned by the black-hole spin (x-axis) and the accretion rate in units of the Eddington rate (y-axis). Non-jetted (radio-quiet) objects populate mainly the low-black-hole-spin regime, whereas galactic nuclei in the high-spin domain are thought to exhibit a relativistic jet and are thus called jetted AGN (or radio-loud AGN).

If non-jetted AGN accrete matter at a low rate, the galactic nucleus is inactive, like in the case of our Milky Way's central supermassive black hole, Sagittarius A*. If they have at least a moderate accretion rate, they are active and observable as a Seyfert galaxy. Seyfert galaxies of type 1 are those objects seen at small viewing angles (more face-on) with well-known examples being Messier 77 or NGC 3147, while type 2 Seyfert galaxies, like for example NGC 1097, are those that are observed at higher viewing angle (more edge-on), such that the dusty, equatorial torus obscures the central engine for this line of sight. At very high accretion rate or very low viewing angle, low-spin AGN are sometimes called quasi-stellar objects.

Jetted AGN with high accretion rate appear as flat-spectrum radio quasars if they are seen face-on. An archetypical example is the long-known object 3C 279. If jetted AGN with high accretion rate are not seen face-on, they are called Fanaroff-Riley type II radio galaxies, like for example the famous Cygnus A radio galaxy. These can, again depending on the viewing angle, appear as broad-line radio galaxies (type 1 objects) or as narrow-line radio galaxies (type 2 objects). At low accretion rate, jetted AGN are appearing as BL Lacertae objects if seen face-on. Famous examples for BL Lacertae objects are the eponymous BL Lacertae itself or the prominent TeV emitter Markarian 501. Jetted AGN with low accretion rate are classified as Fanaroff-Riley I radio galaxies if seen more edge-on. A typical example for this AGN subclass is Messier 84.

Jetted AGN that are seen at small viewing angles are comprisingly called blazars.

Remark however that it is still unclear whether the physical parameter corresponding to the x-axis is really the angular momentum of the central supermassive black hole, or perhaps another quantity like its mass or even a combination of several parameters. Notice also that in AGN classification there are far more classes and subclasses, depending on more refined observational properties, and ambiguities as well as overlaps. This figure was motivated by the sketch by Prof. Dr. Charles Dermer appearing in DOI 10.1016/j.crhy.2016.04.004.