thumb|right|The [[Circinus Galaxy, a Type II Seyfert galaxy]]

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasar host galaxies. They have quasar-like nuclei (very luminous sources of electromagnetic radiation that are outside of our own galaxy) with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines,]]

In the 1960s and 1970s, research to further understand the properties of Seyfert galaxies was carried out. A few direct measurements of the actual sizes of Seyfert nuclei were taken, and it was established that the emission lines in NGC 1068 were produced in a region over a thousand light years in diameter.]]

A lower limit to the mass of the central black hole can be calculated using the Eddington luminosity.

thumb|A model 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.

Emissions

The emission lines seen on the spectrum of a Seyfert galaxy may come from the surface of the accretion disc itself, or may come from clouds of gas illuminated by the central engine in an ionization cone. The exact geometry of the emitting region is difficult to determine due to poor resolution of the galactic center. However, each part of the accretion disc has a different velocity relative to our line of sight, and the faster the gas is rotating around the black hole, the broader the emission line will be. Similarly, an illuminated disc wind also has a position-dependent velocity.]]

In some cases, the spectra show both broad and narrow permitted lines, which is why they are classified as an intermediate type between Type I and Type II, such as Type 1.5 Seyfert. The spectra of some of these galaxies have changed from Type 1.5 to Type II in a matter of a few years. However, the characteristic broad Hα emission line has rarely, if ever, disappeared.]]

Type&nbsp;I Seyferts are very bright sources of ultraviolet light and X-rays in addition to the visible light coming from their cores. They have two sets of emission lines on their spectra: narrow lines with widths (measured in velocity units) of several hundred km/s, and broad lines with widths up to 10<sup>4</sup> km/s.]]

Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at infrared wavelengths. In some Type&nbsp;II Seyfert galaxies, analysis with a technique called spectro-polarimetry (spectroscopy of polarised light component) revealed obscured Type&nbsp;I regions. In the case of NGC 1068, nuclear light reflected off a dust cloud was measured, which led scientists to believe in the presence of an obscuring dust torus around a bright continuum and broad emission line nucleus. When the galaxy is viewed from the side, the nucleus is indirectly observed through reflection by gas and dust above and below the torus. This reflection causes the polarisation.

  • Messier 64 (NGC 4826), with two counter-rotating disks that are approximately equal in mass
  • Messier 66 (NGC 3627), a part of the Leo Triplet
  • Messier 77 (NGC 1068), one of the first Seyfert galaxies classified which has a water vapor megamaser in its nucleus seen by 22 GHz line of ortho-H<sub>2</sub>O
  • NGC 262, an example of a galaxy with an extended gaseous H I halo
  • NGC 1566, one of the first Seyfert galaxies classified

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  • Active Galaxies and Quasars at NASA.gov
  • Seyfert Galaxies at SEDS.org
  • Seyfert Galaxies at ESA.int