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Encyclopedia > Active galaxy

An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, UV, X-ray and gamma rays. This article is about a celestial body. ... The Pleiades star cluster A star is any massive gaseous body in outer space, just like the Sun. ... In astronomy, the interstellar medium (or ISM) is the matter and energy content that exists between the stars (or their immediate circumstellar environment) within a galaxy. ... The electromagnetic spectrum encompasses all possible wavelengths of electromagnetic radiation. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... This article is about electromagnetic radiation. ...

A 5000 light-year (50 Em or 30,000,000,000,000,000 mile) long jet is ejected from active galaxy M87 (the yellow ball at top left). Electrons are ejected outward at near light-speed, emitting eerie blue light.
A 5000 light-year (50 Em or 30,000,000,000,000,000 mile) long jet is ejected from active galaxy M87 (the yellow ball at top left). Electrons are ejected outward at near light-speed, emitting eerie blue light.

Frequently, the abbreviation AGN (Active Galactic Nuclei) is used, since most active galaxies appear to be powered by a compact region in the galactic centre. Some of these compact regions emit jets of matter that can extend for very large distances, powering extended structures (such as radio galaxies and radio-loud quasars). But in all cases the active nucleus or central engine is the fundamental source of energy. Download high resolution version (611x638, 41 KB)from http://hubblesite. ... Download high resolution version (611x638, 41 KB)from http://hubblesite. ... Relativistic Jet. ...


The standard theoretical model is that the energy is generated by matter falling onto a supermassive black hole of between 106 and 109 solar masses. As the material falls into the black hole, angular momentum causes the material to flatten into an accretion disk. Frictional heating causes the infalling material to turn into plasma, and this charged moving material produces a strong magnetic field via the alpha mechanism. Frequently, one observes jets emanating from the accretion disk, although the mechanism of formation of those jets is poorly understood. The accretion mechanism is highly efficient at turning matter into energy, and can convert almost 50% of the mass-energy of an object into energy as compared with only a few percent with nuclear fusion. Top: artists conception of a supermassive black hole drawing material from a nearby star. ... An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ... Current flowing through a wire produces a magnetic field (M) around the wire. ... The deuterium-tritium fusion reaction is considered the most promising for producing fusion power. ...


It is believed that when the black hole has eaten all of the gas and dust in its neighborhood that the active galactic nucleus ceases to emit large amounts of radiation and becomes a normal galaxy. This model is supported by what appears to be a quiet supermassive black hole in the center of the Milky Way, and in other nearby galaxies, and also nicely explains why quasars appear to have been much more common in the early universe, when more fuel was available. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos) is the galaxy in which the Earth is found. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ...


This model also explains the different types of active galactic nuclei, which are believed to all be due to the same type of source, but can appear quite different depending on the angle the source makes to the earth, and the amount of gas and dust available to be fed into the black hole.


Types of active galaxies

Seyferts, quasars and blazars are the main types of AGNs that emit high-energy radiation (X-rays). Quasars, in particular, are thought to be the most consistently luminous objects in the known universe. Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... A blazar is a galaxy with a very compact and highly variable energy source at the center of the host galaxy. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...


Radio galaxies are a heterogeneous group of radio-emitting objects often called Fanaroff and Riley (FR) galaxies after Bernie Fanaroff and Julia Riley. Most of them have huge symmetrical lobes from which the greater part of the radio emission comes. Some of them show a jet or jets (the most famous example being the giant galaxy M87 in the Virgo cluster) coming directly from the nucleus and going to the lobes. The jets are believed to be the visible manifestations of the beam of high-energy particles that power the lobes. The radio emission is synchrotron radiation, implying that the radio lobes and jets contain relativistic electrons and magnetic fields. Julia Riley, a Fellow of Girton College, works at the Cavendish Astrophysics Group, University of Cambridge in the study radio astronomy. ... The jet emitted by M87 in this image is thought to be caused by a supermassive black hole at the galaxys center. ... A sky field near some of the brighter galaxies in the Virgo cluster. ... Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of relativistic electrons (i. ... Properties The electron is a subatomic particle. ... Current flowing through a wire produces a magnetic field (M) around the wire. ...


The "Fanaroff and Riley" radio galaxies are divided into two classes: Fanaroff and Riley Class I (FR I), and Fanaroff and Riley Class II FR II). The distinction was originally made based on the morphology of the radio emission (the type was determined by the distance between the brightest points in the radio emission). Now, the distinction is based on the energy transport mechanism in the radio source. FR I objects typically exhibit subsonic jets and a meandering, diffuse cloud of emission. FR II objects have supersonic jets that terminate in hotspots and result in backflows.


Some of the different types of active galaxy are linked by Unified models in which they are really the same class of object seen at different viewing angles, with relativistic beaming and dust obscuration causing the observational differences. The two main unified models link the different classes of Seyferts and radio galaxies, quasars and blazars. Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... A blazar is a galaxy with a very compact and highly variable energy source at the center of the host galaxy. ...


See also


  Results from FactBites:
 
Active Galaxies and Quasars - Introduction (1211 words)
Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy.
Models of active galaxies concentrate on the possibility of a supermassive fl hole which lies at the center of the galaxy.
Active galaxies are intensely studied at all wavelengths.
Seyfert Galaxies & Active Galaxies (406 words)
The nuclear regions are so luminous that they account for much of the galaxy's light, often having the appearance of stars in photographic plates because they so strongly outshine their spiral host galaxies [Seyfert, 1943].
The defining characteristic of active galaxies is the presence of an extremely energetic process in their nuclei.
Their radio emissions can be so powerful that the brightest objects in the radio sky are active galaxies, despite the fact that they are also some of the farthest objects known.
  More results at FactBites »

 
 

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