About 0.5% of galaxies have Active Galactic Nuclei (AGNs), the most famous of which are quasars, for which there are dozens of different categories under often overlapping classification schemes. A new review paper considers the "AGN Zoo" and tries to make sense of the relatively modest number of parameters that sum up the variation in AGN types. The body text of the paper begins by introducing the concept of an AGN (most citations omitted).
The review paper and its abstract are as follows:The discovery of quasars (Schmidt, 1963) opened up a whole new branch of astronomy. Twenty years earlier Seyfert (1943) had reported the presence of broad and strong emission lines in the nuclei of six spiral nebulae (including some by now “classical” AGN, like NGC 1068 and NGC 4151). However, his work remained largely ignored until Baade & Minkowski (1954) pointed out the similarities between the spectra of the galaxies studied by Seyfert and that of the galaxy they had associated with the Cygnus A radio source.As implicit in the name, AGN are stronger emitters than the nuclei of “normal” galaxies. This “extra” component is unrelated to the nuclear fusion powering stars and is now universally accepted to be connected instead to the presence of an actively accreting central supermassive (greater than equal to 10^6 stellar masses) black hole (SMBH).AGN have many interesting properties. These include: (1) very high luminosities (up to Lbol ≈ 1048 erg s−1 ), which make them the most powerful non-explosive sources in the Universe and therefore visible up to very high redshifts (currently z = 7.1); (2) small emitting regions in most bands, of the order of a milliparsec, as inferred from their rapid variability, implying high energy densities; (3) strong evolution of their luminosity functions (LFs); (4) detectable emission covering the whole electromagnetic spectrum.
Active Galactic Nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different "flavours" in the literature that now comprise a complex and confusing AGN "zoo". It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN, and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their "big picture" through observations in each electromagnetic band from radio to gamma-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.P. Padovani, et al. "Active Galactic Nuclei: what's in a name?" (July 22, 2017).