The Physics and Evolution of Active Galactic Nuclei - 9781107021518

"Observations of active galactic nuclei The names "active galaxies" and "active galactic nuclei" (AGN) are related to the main feature that distinguishes these objects from inactive (normal or regular) galaxies: the presence of supermassive accreting black holes (BHs) in their centers. As of 2011, there were approximately a million known sources of this type selected by their color and several hundred thousand by basic spectroscopy and accurate redshifts. It is estimated that in the local universe, at z < 0.1, about 1 out of 50 galaxies contains a fast-accreting supermassive BH, and about 1 in 3 contains a slowly accreting supermassive BH. Detailed studies of large samples of AGN, and the understanding of their connection with inactive galaxies and theirredshift evolution, started in the late 1970s, long after the discovery of the first quasi-stellar objects (hereinafter quasars or QSOs) in the early 1960s. Although all objects containing active supermassive BHs are now referred to as AGN, various othernames, relics from the 1960s, 1970s, and even later, are still being used. Some of the names that appear occasionally in the literature, such as "Seyfert 1 galaxies" and "Seyfert 2 galaxies," in honor of Seyfert, who observed the first few galaxies of this type in the late 1940s (see Chapter 6 for a detailed discussion of the various groups), are the result of an early confusion between different sources that are now known to have similar properties"--