Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. The light from the central nucleus varies in less than a year, which implies that the emitting region must be less than one light year across. They are named for the astronomer Carl Seyfert, who studied them extensively in the 1940s. They are a subclass of active galactic nuclei.

Seyfert Type 2 spiral galaxy in Circinus

Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4000 km/sec, and are believed to originate in an accretion disk surrounding the central black hole. Each part of the accretion disk has a different velocity relative to our line of sight, and the faster the gas is rotating about the black hole, the broader the line will be. The narrow lines are believed to originate from the outer part of the disk where the rotational velocity is lower, while the broad lines originate closer to the black hole. This is confirmed by the fact that the narrow lines do not vary detectably, which implies that the emitting region is large, contrary to the broad lines which can vary on relatively short timescales. Seyfert galaxies also show strong emission in the radio, infrared, ultraviolet, and X-rays parts of the spectrum.

Seyferts were first classified as Type 1 or 2, depending upon whether the spectra show both narrow and broad emission lines, or only narrow lines. They are now given a fractional classification depending upon the relative strengths of the narrow and broad components (e.g. Type 1.5 or Type 1.9). The narrow and broad components are believed to both originate from the accretion disk, but in Type 2 Seyferts it is believed that the broad component is obscured by dust and/or by our viewing angle on the galaxy. In some Type 2 Seyfert galaxies, the broad component can be observed in polarized light; it is believed that light from the broad-line region is scattered by a hot, gaseous halo surrounding the nucleus, allowing us to view it indirectly. This effect was first discovered in the Type 2 Seyfert NGC 1068.