Active Galactic Nuclei

Abstract

In what is defined as Active Galactic Nuclei, the stellar contribution to the emitted light is generally less than (or exceptionally equal to) half the total emitted power. Even if a spectrum of an active galaxy was taken as early as 1908, Seyfert was the first astronomer to understand thatactive galaxiesdo form a class. Seyfert’s original work (1943) listed 12 galaxies, defined at that time asnormal galaxies with high surface brightness nuclei.Spectroscopy then revealed most characteristic properties of Seyfert galaxies, the standard of AGN: acontinuum with both a UV and an IR excess(with respect to usual galaxy continua), together with numerous broad emission lines with high degree of ionization and excitation. Other characteristics of AGN are the following: they have in average large redshifts, some are radio loud, almost all are X-ray emitters, a majority seems to emit in the high-energy range, γ-ray and TeV bands, and many of them present time variability. The discrimination between active galaxies and quasars is mostly historical, due to the very different morphologies of the first Seyferts and quasars. Furthermore, the status of AGN remained unclear for a long time, due to the mixture of morphological and spectroscopic criteria used in many of their first classifications. Since that time, and essentially based on the similarity of their spectra and of most of their properties which vary continuously from the low luminosity less active galaxies to the most luminous quasars.