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Active Galactic Nuclei

In some galaxies, known as "active galactic nuclei" (AGN), the nucleus (or central core) produces more radiation than the entire rest of the galaxy! Quasars are very distant AGN - the most distant quasars mark an epoch when the universe was less than a billion years old and a sixth of its current size. In some cases, the size of the AGN is smaller than the size of our solar system. Current theory suggests that there is a supermassive black hole (millions of times the mass of the sun) at the center of AGN. (See Introduction to Active Galactic Nuclei.)

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BL Lacertae Last Updated on 2008-04-15 00:00:00 BL Lacertae (BL Lac or S4 2200+420) was known to be variable in the visible part of the electromagnetic spectrum from as early as 1929, and because of its stellar appearance it was originally thought to be a VARIABLE STAR. It is located in the constellation Lacerta, the lizard (RA (J2000) = 22h 02m 43.29s, DEC (J2000) = +42o 16’ 39.98”). Observations in the late 1960s showed it to have highly variable radio emission as well. BL Lac was found to be an extragalactic object with the measurement of its redshift (z = 0.069, based on the detection of very weak emission lines) in the early 1970s. During the same period, it was also understood to be an unusual type of ACTIVE GALACTIC NUCLEUS (AGN). Thus, BL Lacertae is the prototype of the class of AGN known as BL LACERTAE OBJECTS (also called BL Lacs). Collectively with some quasars, such as 3C273 and 3C279, they are known as... More »
Quasar Microlensing Last Updated on 2007-12-25 00:00:00 The Einstein gravitational theory predicts that light and all other wavelengths of electromagnetic radiation are bent in the vicinity of any massive body, and it is of course everyday experience that the bending is neglegible in everyday experience. But, in astronomy where vast distances are involved, the bending can be sufficient to distort and displace the image, and in extreme cases can produce multiple images of a single distant source. This phenomenon is called gravitational lensing, and in extreme cases multiple images separated by a few arc-seconds on the sky are found for distant quasars lensed by a foreground galaxy with the mass of the Milky Way galaxy. Since discovery of the first such object by Dennis Walsh in 1979, 80 such objects have been recognized at optical and radio wavelengths. Formation of multiple images is always accompanied by magnification of the source image... More »
Active Galactic Nuclei (AGN) Last Updated on 2007-11-08 00:00:00 Galaxies are now known to contain Supermassive Black Holes in their centers. In most galaxies (perhaps 90 percent) the central black hole has very little observable effect on the galaxy. In the remaining about 10 percent, a supply of gas becomes available, perhaps in Interacting Galaxies and Mergers. When the gas sinks to within a distance of a few light years, the black hole pulls gas into an accretion disk roughly the size of our solar system. The gas becomes heated to high temperatures as it falls into the accretion disk. The accretion disk can be very luminous and in the brightest sources (called quasars) can out shine all the stars in the galaxy. The objects in which the black hole is accreting gas in this way are said to have Active Galactic Nuclei (AGN). In addition to the photons from the accretion disk, AGN may also generate winds or outflows. In fact outflows seem to be a... More »