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Seyfert galaxy(see -fert. sÿ -) A galaxy with an exceptionally bright nucleus, first described by Carl Seyfert in 1943. Over 150 Seyfert galaxies have now been identified and the vast majority of these are otherwise normal nearby spiral galaxies. Seyferts have substantial nonthermal emission in their nuclei. The nuclei emit radiation at optical wavelengths (by definition) but most of their output is in the infrared; they are also strong X-ray and ultraviolet sources but are not often powerful at radio wavelengths. Their output can vary on a timescale of months. This indicates that the energy source in Seyfert galaxies must be a few light-months across and astronomers believe that Seyferts are a less powerful example (by a factor of 100) of the quasar phenomenon.
A region of hot ionized gas clouds surrounds the Seyfert nucleus, giving rise to an optical spectrum rich in emission lines. Type 1 Seyferts exhibit both broad Balmer line emission of hydrogen and narrow lines of ionized metals in a manner directly analogous to the broad and narrow line regions (BLR and NLR) of quasars. Type 2 Seyferts show only narrow lines of all species.
The distinction between the two types can be modeled as an effect of inclination. In type 1 Seyferts the nucleus of the galaxy can be viewed directly and thus the central BLR that surrounds the black hole is seen. In type 2 Seyferts, however, the view of the very central regions is obscured by a thick molecular torus that lies outside the BLR and so only the emission from the NLR is seen. Observations of faint reflected broad lines in some type 2s support this unification hypothesis.
Some 10% of giant spiral galaxies are Seyferts. As Seyfert activity is probably only a transitory period in the life of a galaxy, then it is possible that all giant spiral galaxies, including our own, spend 10% of their lives in a Seyfert phase.