What is Seyfert galaxy?
[[File:circinus.galaxy.750pix.jpg|thumb|250px|The [[Circinus Galaxy]], a Type II Seyfert galaxy]] [[File:Active_Galactic_Nucleus_Model.png|thumb|250px|A diagram of an [[active galactic nucleus]]. The central black hole is surrounded by an accretion disc, which is surrounded by a torus. The broad line region and narrow line emission region are shown, as well as jets coming out of the nucleus]] '''Seyfert galaxies''' are a type of [[galaxy]]. They are named after [[Carl Seyfert]] who first described them in 1943.<ref name=Seyfert>Seyfert, Carl K. 1943. Nuclear emission in spiral nebulae. The ''Astrophysical Journal''. '''97''': 28–40. [http://adsabs.harvard.edu/doi/10.1086/144488]</ref> These galaxies are one of the two main types of [[active galactic nucleus|active galaxies]]. The other large group are [[quasar]]s. Seyfert galaxies have quasar-like nuclei: they are very distant [[luminous]] sources of [[electromagnetic radiation]]. Their very high surface brightness has [[spectrum|spectra]] with strong, high-[[ionisation]] [[emission spectrum|emission lines]].<ref name=Petersen>Peterson, Bradley M. 1997. ''An introduction to active galactic nuclei''. Cambridge University Press. {{ISBN|978-0-521-47911-0}} [http://ned.ipac.caltech.edu/level5/Cambridge/frames.html]</ref> However, unlike quasars, their host galaxies are clearly visible.<ref name=Bulgaria>Petrov G.T. (ed) 2004. ''Active galaxy nuclei''. Bulgarian Academy of Sciences/Institute of Astronomy [http://www.astro.bas.bg/~petrov/galaxies_files/agn.html].</ref> Seyfert galaxies are about 10% of all galaxies,<ref name=Maiolino>Maiolino R. & Rieke G.H. 1995. Low-luminosity and obscured Seyfert nuclei in nearby galaxies. ''The Astrophysical Journal''. '''454''': 95–105. [http://adsabs.harvard.edu/abs/1995ApJ...454...95M]</ref> and are some of the most intensely studied objects in [[astronomy]]. They are thought to be powered by the same phenomena as quasars, although they are closer and less luminous than quasars. These galaxies have [[supermassive black hole