Active Galaxies

Active Galaxies
Home The Scientific Method The Heavens History Light & Telescopes The Solar System The Earth & Moon Terrestrial Planets The Jupiter System The Jovian Planets Solar System Leftovers The Sun Stellar Parameters Sun-Like Stars High Mass Stars The Milky Way Normal Galaxies Active Galaxies Cosmology Text only © 1998 - 2001 Paul J. Marquard. Images may be copyrighted by many different sources. This web site funded through the NASA Space Grant College and Fellowship Program and the Wyoming Space Grant Planetary & Space Science Center, NASA Grant #NGT40008. If you have comments about these pages, I would be happy to hear them. Please email me at marquard@acad.cc.whecn.edu.		In 1936 a radio telescope was built in the backyard of an amateur astronomer. This was the beginning of radio astronomy. Three objects were detected with this radio telescope. One was the center of the galaxy, another was a supernova remnant within the Milky Way. But the third source (Cygnus A) was very unusual and not identified for almost ten years. Cygnus A turned out to be an unusual galaxy. Unusual in the sense that it emitted a large quantity of radio radiation and that it spectral lines were emission lines. It also showed a redshift indicating it to be one of the farthest objects in the known galaxy of the time. In 1960 another peculiar radio source was discovered, and this one turned out to be a "star", also with emission lines. But it would be three years before the emission lines were discovered to be due to H and were extremely redshifted. This quasi-stellar radio source would become known as a "quasar". The extreme redshift of the spectral lines was another indication of a very distant object. Since the discovery of that first quasar, more than 10,000 quasars have been discovered. Their characteristics include a star-like appearance, an extreme redshift (some exceeding 90 percent the speed of light), and an emission spectra. Only about 10 percent of the known quasars show radio emission. A Quasar Portrait Gallery Credit: J. Bahcall (IAS, Princeton), M. Disney (Univ. Wales), NASA The extreme distance to quasars indicates that their brightness may be more than 1000 times brighter than the whole Milky Way galaxy. It is unlikely that this source of radiation is blackbody radiation. We now believe this to be synchrotron radiation. The synchrotron radiation indicates a very active object. It is now believed that this very active object is in the center of a galaxy. We will soon discuss the source of this immense activity. There seems to be a vast difference in activity when comparing quasars to regular galaxies. This difference can be filled in with active galaxies such as Seyfert galaxies and radio galaxies. These galaxies seem to have an activity similar to a quasar but on a smaller scale. Photographs tend to show spiral and elliptical galaxies associated with Seyfert galaxies and radio galaxies respectively. Another source of extreme luminosity is the blazar. These objects also exhibits synchrotron radiation and are believed to be the core of a galaxy. The collection of quasars, blazars, Seyfert galaxies, and radio galaxies are all referred to as active galaxies. It is believed that the center of these active galaxies contains a supermassive black hole. Surrounding this black hole is a large accretion disk. As the matter from the accretion disk falls into the black hole the gravitational energy is used to excite the gases in the nuclei of the galaxies. Evidence for supermassive black holes exists in the core of our own galaxy and the nucleus of the Andromeda galaxy. There is also some indication that quasars, blazars, and radio galaxies are basically the same thing viewed from a slightly different angle. Black Holes in Galactic Centers Credit: D. Richstone (U. Mich) et al., HST, NASA