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Seyferts, Agn's and Supermassive Black Holes

by WKT

July 14, 2003 ::
NGC 1068
NGC 1068
Messier 77, also known as NGC 1068, is the nearest (50 million light years distant) and brightest example of a class of galaxies called Seyfert galaxies. They are named after Carl Seyfert, an astronomer who used the 100-inch telescope on Mt. Wilson near Los Angeles in the early 1940's to make detailed studies of the central regions of some spiral galaxies.

Carl Seyfert with Telescope
Carl Seyfert at the 24 inch telescope at Vanderbilt University. After his death, the telescope was named in his memory.
(Credit: U. Vanderbilt)
Seyfert's study focused on six galaxies including NGC 1068 that had evidence of unusual activity in their nuclei. The light from these nuclei, in contrast to the absorption-line spectra of starlight from the outer spiral arms of the galaxies, exhibited emission-line spectra characteristic of hot ionized gas clouds. His observations showed that the hot gas clouds were moving rapidly out of the galaxies' nuclei at speeds ranging from several hundred thousand miles per hour up to millions of miles per hour.

Further research has shown that about 2% of all spiral galaxies can be classified as Seyfert galaxies. Their nuclei are now included in a broad category called Active Galactic Nuclei, or AGN's. Powerful radiation from AGN's is observed at optical, radio, infrared, X-ray and gamma-ray wavelengths. A quasar or blazar is considered to be an extreme example of an AGN where the active nucleus outshines the entire galaxy!

Illustration of an AGN
Illustration of an AGN
The pioneering work of Seyfert, who died in an automobile accident in 1960 at the age of 49, provided the first evidence for violent, explosive events in the nuclei of galaxies and set the stage for one of the most important discoveries of astrophysics - the existence of supermassive black holes. The generally accepted view today is that central supermassive black holes are responsible for this radical activity.

Illustration of Black Hole with Accretion Disk and Torus (Face-on View) Illustration of Black Hole with Accretion Disk and Torus (Side View)
Illustrations of Black Hole with Accretion Disk and Torus
The differences in the various AGN's are thought to be due to a variety of factors which could relate to both the evolutionary stage of the AGN and the viewing geometry. These include the size of the central supermassive black hole, the amount of gas available to feed it, and whether or not the view of the black hole is obscured by a disk or torus of gas and dust around it.

Chandra Image of Sgr A* with Labels
Chandra Image of Sgr A* with Labels
The central black hole in NGC 1068 has a mass of about 10 million solar masses. This is a few times more massive than Sgr A*, the supermassive black hole at the center of our Milky Way galaxy. Although the Milky Way is not now a Seyfert galaxy, it may have been one in the past, or it could flare up hundreds of millions of years in the future, if one of its nearby satellite galaxies gets pulled into its nucleus.

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