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Topic: Active galactic nucleus

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	Active galactic nucleus - Wikipedia, the free encyclopedia
		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 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 fl hole.
		Radio galaxies are active galaxies with strong, extended radio emission, powered by relativistic jets from the active nucleus.
	en.wikipedia.org /wiki/Active_galaxy (560 words)

	active galactic nucleus (AGN)
		AGN are found at the heart of active galaxies, including quasars, Seyfert galaxies, blazars, and radio galaxies.
		Since a fl hole, by definition, emits nothing, the radiation from an AGN is believed to come from material heated to several million degrees in an accretion disk before tumbling into the fl hole or, in some cases, being shot away in twin jets along the central engine’s spin axis.
		Within each is a broad range of luminosity, from AGN so weak that they are barely detectable against the light from the central stars of their host galaxy to quasars that are more than 100 times brighter than all the stars in their hosts put together.
	www.daviddarling.info /encyclopedia/A/AGN.html (740 words)

	Active Galaxies and Quasars - Introduction
		Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy.
		Active galaxies are intensely studied at all wavelengths.
		The AGN's observed at higher energies form a subclass of AGNs known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that what we observe is primarily emission from the jet region.
	imagine.gsfc.nasa.gov /docs/science/know_l1/active_galaxies.html (1213 words)

	Active Galaxies and Quasars - Introduction and Glossary
		The recognition of active galactic nuclei may be conveniently traced to three discoveries, which taught us distinct aspects of the phenomenon as well as how they might be linked.
		A new wrinkle in the AGN picture was added in the late 1970s, with the identification of a few enigmatic objects from variable-star catalogs as highly variable nuclei of distant galaxies.
		Some of these have been shown to be low-luminosity active galactic nuclei, perhaps an extension of Seyfert activity to the lowest levels and implying that the whole phenomenon of nuclear activity occurs in a significant fraction of bright galaxies.
	www.astr.ua.edu /keel/agn/text.html (3696 words)

	Sombrero Galaxy - Wikipedia, the free encyclopedia
		Infrared spectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation actvity.
		This emission is actually quite common for active galactic nuclei.
		"Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)".
	en.wikipedia.org /wiki/Sombrero_galaxy (1320 words)

	BL Lac object - Wikipedia, the free encyclopedia
		At variance with other type of active galactic nucleus BL Lacs are characterized by rapid and large amplitude flux variability and significant optical olarization.
		In the unified scheme of radio loud active galactic nucleus the observed nuclear phenomenology of BL Lacs is interpreted as due to the effects of the relativistic jet that is pointing towards the observer.
		From the point of AGN classification BL Lacs are a subtype of blazar.
	en.wikipedia.org /wiki/BL_Lac_object (252 words)

	NGC5128text (Site not responding. Last check: 2007-10-25)
		It is also the nearest of the giant radio galaxies, possessing an active galactic nucleus (AGN) and optically one of the most luminous galaxies in the sky.
		Active galaxies are distinguished by their prodigious energy output which cannot be explained by their stellar populations and must have another source.
		Active galaxies have in common an "Active Galactic Nucleus (AGN)" which is believed responsible for their prodigious energy output.
	www.robgendlerastropics.com /NGC5128text.html (619 words)

	Astronomy glossary
		As a comet's nucleus is usually quite small, it is not able to retain its coma for long periods of time, and the coma material gradually drifts away into space (helped out by the solar wind).
		A negatively charge elementary particle that typically resides outside the nucleus of an atom but is bound to it by electromagnetic forces.
		Note that the true comet nucleus is rarely, if ever, directly observed from the earth because of the large amount of gas and dust that is ever-present in the inner coma close to the nucleus, serving to hide the true nucleus' surface.
	mysite.wanadoo-members.co.uk /blobrana/database/key.html (17192 words)

	M58text (Site not responding. Last check: 2007-10-25)
		A subset of galaxies display peculiar spectra from their nucleus that is either nonstellar in origin or from hot and massive population I stars not normally found in the central region.
		Active galaxies (galaxies containing an active galactic nucleus or AGN) are the classic nonstellar emitters as their central mass accreting fl holes release prodigious amounts of energy at a large range of wavelengths.
		As is typical of Seyfert galaxies, M58 possesses a central jet phenomenon in its nucleus presumably due to a mass accreting central fl hole.
	www.robgendlerastropics.com /M58text.html (573 words)

	[No title] (Site not responding. Last check: 2007-10-25)
		FOR RELEASE: 9:20am CST, January 6, 1999 International team discovers a gas bar feeding an active galactic nucleus Astronomers announced today the discovery of a gaseous bar that appears to be "feeding" a supermassive fl hole located in the nucleus of the active galaxy, Circinus.
		Active galaxies are among the brightest objects in the sky.
		Additional observations of radiation from molecular hydrogen, taken with the AAT, indicated that the gas along the leading edge of the bar, the edge that lies in the direction of the motion of the rotating bar, is flowing into the center of Circinus.
	nicmos.as.arizona.edu /science/pressrelease/990106.txt (916 words)

	Linné on line - Active Galactic Nuclei
		Galaxies with active nuclei are similar to spiral galaxies.
		The nucleus is typically much heavier than in spiral galaxies and pulls material from the galactic disc.
		The amount of energy that is converted in active galactic nuclei is enormous.
	www.linnaeus.uu.se /online/phy/macrocosmos/agn.html (214 words)

	HubbleSite - Glossary
		The brightness of an active galactic nucleus is thought to come from an accretion disk around a supermassive fl hole.
		A galaxy possessing an active galactic nucleus at its center.
		The brightest type of active galactic nucleus, believed to be powered by a supermassive fl hole.
	hubblesite.org /reference_desk/glossary/galaxies.shtml (1604 words)

	Wikinfo \| Active galaxy (Site not responding. Last check: 2007-10-25)
		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.
		Frequently, the abbreviation AGN (Active Galactic Nuclei) is used, since all active galaxies appear to be powered by a compact region in the galactic centre.
		The standard model is that the energy is generated by matter falling onto a supermassive fl hole of between 1 million and 1 billion solar masses.
	www.wikinfo.org /wiki.php?title=Active_galaxy (712 words)

	Department of Physics and Astronomy
		Above is a model of the ultraviolet and optical line emission due to singly and doubly ionized iron from the Broad-Line region of an Active Galaxy.
		Such models can be used to perform quantitative spectral analysis of the emission spectra of active galaxies in order to discover the nature and basic physical properties of the emitting gas, as well as the abundance of the chemical elements.
		"Active Galactic" is a generic term for a wide variety of energetic phenomena in the centres of galaxies.
	www.astro.uwo.ca /research/galaxies.html (257 words)

	Hubble Deep Field Encyclopedia Article @ LaunchBase.com (Launch Base) (Site not responding. Last check: 2007-10-25)
		Once Hubble's corrective optics were shown to be performing well, Robert Williams, the then director of the Space Telescope Science Institute, decided to devote a substantial fraction of his DD time during 1995 to the study of distant galaxies.
		It had to be at a high galactic latitude, because dust and obscuring matter in the plane of the Milky Way's disc prevents observations of distant galaxies.
		In all, the HDF is thought to contain fewer than ten galactic foreground stars; by far the majority of objects in the field are distant galaxies.
	www.launchbase.com /encyclopedia/Hubble_Deep_Field (2076 words)

	CERN Courier - Closest view of active galac - IOP Publishing - article
		NGC 1068 was among a group of 12 galaxies identified by Carl Seyfert in 1943 as having a nucleus with a spectrum showing peculiar emission lines from highly ionized atoms.
		The active nucleus of Seyfert galaxies and quasars emits radiation over the whole electromagnetic spectrum, from radio to X-rays and even gamma rays.
		While the yellowish nucleus of stars was only mildly affected by the collision, the gas rushing outwards has formed a ring, like ripples in a pond after a rock has been thrown in.
	www.cerncourier.com /main/article/44/5/11 (649 words)

	Double Radio Sources Associated with Active Galactic Nuclei
		The process that launches the plasma jets is thought to be the winding up of magnetic fields in an accretion disk that forms around a massive fl hole in the galactic nucleus.
		The basic scheme is sketched in the HEASARC animation at the left, which zooms in from a distant side view to look down onto the fl hole in a close-up from near the rotation axis of the disk.
		The outflow speeds in AGN are thought to be initially supersonic (i.e., the flows are faster than the speed of sound in the jet plasma).
	www.cv.nrao.edu /~abridle/dragnparts.htm (1202 words)

	Active Galactic Nuclei
		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.
		Based on X-ray (and other) observations, a good guess is that the power source in AGN is a supermassive fl hole.
	heasarc.gsfc.nasa.gov /docs/objects/agn/agntext.html (225 words)

	Print the story
		NGC 1097 is a very moderate example of an Active Galactic Nucleus (AGN), whose emission is thought to arise from matter (gas and stars) falling into oblivion in a central fl hole.
		However, NGC 1097 possesses a comparatively faint nucleus only, and the fl hole in its centre must be on a very strict "diet": only a small amount of gas and stars is apparently being swallowed by the fl hole at any given moment.
		Details from the nucleus and its immediate surroundings are however outshone by the overwhelming stellar light of the galaxy seen as the bright diffuse emission all over the image.
	www.physorg.com /printnews.php?newsid=7296 (1016 words)

	Science Glossary - A
		A galaxy which releases large amounts of energy from its centre, the active galactic nucleus.
		The central engine of an active galaxy probably is a supermassive fl hole.
		Seyfert galaxies, quasars and blazars are active galaxies.
	sci2.esa.int /glossary/glossary_a.html (897 words)

	Active Galaxies
		In ~10% of the AGN, the MBH + accretion disk somehow produce narrow beams of energetic particles and magnetic fields, and eject them outward in opposite directions away from the disk.
		The radio components include: the compact core at the galaxy nucleus, jets, lobes, and a hot spot where the jet slams into the interstellar medium.
		Active Galactic Nuclei observed at high (>100 MeV) energies form a subclass known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that the emission we observe is dominated by phenomena occurring in the jet region.
	imagine.gsfc.nasa.gov /docs/science/know_l2/active_galaxies.html (1045 words)

	M87 (Site not responding. Last check: 2007-10-25)
		M87 is also known as NGC4486, 3C274, and as Virgo A (the strongest radio source in the constellation Virgo) It is one of the first galaxies discovered to have an optical "jet", which is visible faintly as it protrudes through the bright optical emission from the stars in the galaxy.
		The center of the AGN is the bright spot on the left side of the image and the jet appears one-sided, heading off toward the upper right.
		One-sided AGN jets are probably caused by strong beaming of the emission for the side of the jet headed toward the observer.
	cfa-www.harvard.edu /~reid/m87.html (360 words)

	Active Galaxies newsletter: Issue 91 (Site not responding. Last check: 2007-10-25)
		We infer from this gradient that the host galaxy disk is tilted towards the observer in the east: the line-of-sight to the eastern emission-line cone intersects more dust in the plane of the galaxy than that to the western cone.
		From emission-line diagnostics we find that the NLR gas is photoionized by the hidden active galactic nucleus (AGN) continuum and that its density decreases with increasing distance from the center.
		The lack of a significant difference between normal and AGN host galaxies in the Fundamental plane supports the ``Grand Unification" picture wherein AGNs are a transient phase in the evolution of normal galaxies.
	www.jb.man.ac.uk /~rbeswick/agn/html-issue/issue91 (2348 words)

	A lightweight review of middleweight black holes
		Supermassive holes, the central engines in active galactic nuclei, are observed because they are very bright, feeding off of the gas-rich environments in the centers of galaxies.
		A suggestion that these sources are just unusually quiescent active galactic nuclei is significant, since it would be interesting to observe nuclear activity well outside the galactic nucleus.
		The lack of any other evidence for behavior typical of active galactic nuclei, plus the dynamical awkwardness of placing a supermassive fl hole in the suburbs of otherwise normal galaxies lends some credence to the idea that the sources are middleweights.
	www.phys.lsu.edu /mog/mog14/node6.html (1018 words)

	Origin and Evolution of the Universe
		Ongoing massive dust formation within the AGN region spills into bars and spiral arms of visible atomic-molecular dust which centrifugally reshapes the growing mass and density of the galaxy from elliptical into spiral form (Sa to Sc).
		The evidence for the presence of these supermassive fl holes is based on the enormous orbital velocity of stars and gas clouds near the fl hole and the presence of optical jets and radio jets exploding outward from quasars and from the active galactic nuclei of many galaxies.
		Galaxies that have an active nuclear center that produces an energy output equivalent to the entire galaxy within which it is contained have been designated as active galaxies or Active Galactic Nucleus (AGN), and AGNs comprise a small percentage of all galaxies.
	www.origin-of-universe.com (4521 words)

	NGC1068 at VLTI
		The paradigm to explain the large variety of active galaxy nuclei is the existence of a supermassive fl hole, surrounded by a torus of gas and dust, hiding it for certain lines of sight.
		It is the first observation of an active galaxy nucleus by an interferometer in the thermal infra-red.
		The active galaxy nuclei (AGN) indicate the central heart of very active galaxies, thought to shelter a supermassive fl hole, several hundreds of million times the mass of the Sun, surrounded by an accretion disc in which gas falls in spiraling towards the center, thus releasing a gigantic energy.
	www.obspm.fr /actual/nouvelle/may04/n1068.en.shtml (994 words)

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