Seyfert galaxy

A galaxy whose compact nucleus emits intense radiation powered by matter accreting onto a supermassive black hole. Its spectrum shows strong emission lines, while the host galaxy remains comparatively ordinary.

Active galactic nucleus: A compact galactic center whose emission is powered by accretion onto a supermassive black hole. Seyfert nuclei are one relatively nearby, lower-luminosity class of active galactic nucleus.
Seyfert 1 galaxy: A Seyfert galaxy whose spectrum includes broad permitted emission lines as well as narrow lines. Visible broad lines mark nuclei whose broad-line regions are not strongly obscured.
Carl Seyfert: An American astronomer who identified galaxies with unusually bright nuclei and strong emission lines. His 1943 study established the observational class that bears his name.
Emission-line spectroscopy: The analysis of discrete wavelengths emitted by atoms, ions, and molecules in excited gas. Line widths and ratios classify Seyferts and diagnose conditions in their ionized gas.
Accretion disk: A rotating structure of gas and dust that spirals inward around a massive central object. Infalling material heats in the disk and supplies much of the nucleus’s luminosity.
Seyfert 2 galaxy: A Seyfert galaxy whose optical spectrum shows narrow emission lines without directly visible broad permitted lines. Its narrow-line spectrum contrasts with type 1, often because the central regions are obscured.
Seyfert's Sextet: A compact group of galaxies in Serpens named for Carl Seyfert. Despite the shared name, this galaxy group is distinct from the Seyfert galaxy class.
Black hole mass measurement: Methods for estimating a black hole’s mass from its gravitational effects or surrounding emission. Seyfert emission-line variability and gas motions help estimate central black-hole masses.
Broad-line region: Fast-moving gas near an active galactic nucleus that produces broad emission lines. Its high-velocity gas produces the broad permitted lines in Seyfert 1 spectra.
Circinus Galaxy: A nearby spiral galaxy containing an obscured Seyfert nucleus and prominent star-forming regions. It is a close example of a dusty, heavily obscured Seyfert system.

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