Quasars and other active galaxies:
 The home to colossal eruptions


ACTIVE GALAXIES: 

An active galaxy is one whose center emits an abnormally large amount of energy. So far,  astronomers have discovered three types of galaxies with active nuclei: Radio Galaxies, Seyfert Galaxies, and Quasars.


The Very Large Array (VLA) of 27 Radio Telescopes in Socorro, N.M. is a wonderful tool to search for radio signals from these powerful sources.


Radio Galaxies

Thet emit vast amounts of energy in the radio part of the electromagnetic spectrum; sometimes up to millions of times more than a normal galaxy. The emission comes from the galactic core as well as from regions well outside the core.



Cygnus A is a powerful double-lobed radio galaxy. This radio map produced by the VLA radio telescope shows hot spots. Note the long, thin jet of matter flowing from the galaxy to the right-hand lobe. The jet is at least 150 light years - or about 50% larger that the diameter of our own Milky Way. Although not perfectly understood, the hot spots in the two lobes are produced by beams of high speed particles (such as electrons) pushing into the intergalactic gas.

Seyfert Galaxies

These spiral galaxies - named after the American astronomer Carl Seyfert - have cores that emit as much energy (at all wavelengths) as the entire radiation output of our own Milky Way but do not contain well defined radio lobes. A striking feature of these galaxies is that their luminosity can change rapidly, sometimes in a matter of minutes!



About 72 million light-years away in the constellation Pegasus, NGC 7742 is known to be a Seyfert galaxy - a type of active spiral galaxy with a center or nucleus which is very bright at visible wavelengths. Ringed by blue-tinted star forming regions and faintly visible spiral arms, the yolk-yellow center is about 3,000 light-years across. Across the spectrum, the tremendous brightness of Seyferts can change over periods of just days to months and galaxies like NGC 7742 are suspected of harboring massive black holes at their cores. This beautiful color picture is courtesy of the newly inaugurated Hubble SpaceTelescope Heritage Project.


Quasars --

Quasars (or quasi-stellar objetcs) are the most powerful energy emitters known to humanity. The energy output of these active galaxies is so enormous that ordinary mechanisms of energy generation are unable to explain their energy output. The favored explanation, at the moment, is to suppose that one or more supermassive black holes exist, at the core of these galaxies, into which matter is falling. As the matter falls into the black hole the gravitational energy of the material is converted into kinetic energy (heat energy), thereby causing the temperature of the infalling gas to rise to millions of degrees and giving up all sorts of intense radiation, especially X-rays.


Quasars reside in a variety of galaxies, from normal to highly disturbed. When seen through ground-based telescopes, these compact, enigmatic light sources resemble stars, yet they are billions of light-years away and several hundred billion times brighter than normal stars. The previous HST images show examples of different home sites of all quasars. But all the sites must provide the fuel to power these unique light beacons. Astronomers believe that a quasar turns on when a massive black hole at the nucleus of a galaxy feeds on gas and stars. As the matter falls into the black hole, intense radiation is emitted. Eventually, the black hole will stop emitting radiation once it consumes all nearby matter. Then it needs debris from a collision of galaxies or another process to provide more fuel. The column of images on the left represents normal galaxies; the center, colliding galaxies; and the right, peculiar galaxies.


Just like Cookie-Monster devours all cookies in its immediate neighborhood, so do black holes with nearby stars.

The Sombrero Galaxy (M104) in the constellation of Virgo is thought to harbor a billion solar mass black hole lurking at its center. The Hubble Space Telescope recently found evidence that the galaxy M87 (which is about 50 million light years from us) contains a supermassive black hole at its core that may be the size of our solar system!

This HSTelescope image reveals the faint host galaxy that a bright quasar dwells within. The wealth of new detail in this picture helps solve a three-decade old mystery about the true nature of quasars, the most distant and energetic objects in the universe. The HST image shows clearly that the quasar lies in the core of a  galaxy that has a common shape consisting of two spiral arms of stars connected by a bar-like feature. The host galaxy is in a spectacular collision with a dwarf galaxy. The collision apparently fuels  the quasar "engine" at the galaxy center - presumably a massive black hole -- and also  triggers many sites of new  star-formation.

Quasars are  the most distant objects in the universe, and so are among the earliest objects known to have formed in the young universe, more than 12 billion years ago. The most widely accepted notion is that quasars are in galaxies with active, supermassive black holes at their centers. However, because of their enormous distance, the `host' galaxies appear very small and faint, and are very hard to see against the much brighter quasar light at the center. Though a quasar might no be much larger than our solar system it releases as much energy as billions of stars.