What+is+AGN?

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 * What is AGN?**

It is the nucleus, the central core, that produces more radiation than the rest of the galaxy. The current theory on AGN is that there is a super massive black hole at the center of the AGN. -It's a theory because AGN is studied by using **spectroscopy** (we will get to that later on) that is based on electromagnetic radiation (light) and light can not escape a black hole.
 * AGN** stands for **Active Galactic Nuclei.**

Source: http://heasarc.gsfc.nasa.gov/docs/objects/agn/agntext.html


 * Why is it an "active" galaxy?**

Well there is no single specific criteria that all AGN galaxies fall into. However, here are some of the key features of what makes a galaxy active:

This is visible whenever we have a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related component. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds, emitting photons at varying Doppler shifts. These come from more distant cold material, and so are narrower than the broad lines. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation. This can arise both from a jet and from the hot corona of the accretion disc via scattering processes: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is a `soft excess' in the X-ray emission in addition to the power-law component. The origin of the soft excess is not clear at present. This is a result of illumination of cold heavy elements by the X-ray continuum. [|Fluorescence] gives rise to various emission lines, the best-known of which is the iron feature around 6.4 [|keV]. This line may be narrow or broad: [|relativistically broadened iron lines] can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.
 * Nuclear optical continuum emission
 * Nuclear infra-red emission
 * Broad optical emission lines
 * Narrow optical emission lines
 * Radio continuum emission
 * X-ray continuum emission
 * X-ray line emission

Source: http://en.wikipedia.org/wiki/Active_galactic_nuclei


 * Brief History of AGN** by Gregory A. Shields

Astronomers knew early in the twentieth century that some galaxies have emission-line nuclei. However, even the systematic study by Seyfert (1943) was not enough to launch active galactic nuclei (AGN) as a major topic of astronomy. The advances in radio astronomy in the 1950s revealed a new universe of energetic phenomena, and inevitably led to the discovery of quasars. These discoveries demanded the attention of observers and theorists, and AGN have been a subject of intense effort ever since. Only a year after the recognition of the redshifts of 3C 273 and 3C 48 in 1963, the idea of energy production by accretion onto a black hole was advanced. However, acceptance of this idea came slowly, encouraged by the discovery of black hole X-ray sources in our Galaxy and, more recently, supermassive black holes in the center of the Milky Way and other galaxies. Many questions remain as to the formation and fueling of the hole, the geometry of the central regions, the detailed emission mechanisms, the production of jets, and other aspects. The study of AGN will remain a vigorous part of astronomy for the foreseeable future.

Source: http://arxiv.org/abs/astro-ph/9903401

__Elliptical Galaxy__: Elliptical galaxies are very common in our universe. However, they are faint in the radio spectrum because they are mostly cool stars. __Starburst Galaxy__: Some galaxies are currently forming stars at a furious rate, going through a stellar “baby boom.” These galaxies are known as starburst galaxies. __Radio Galaxy__: The term radio galaxy was coined to describe objects that look like normal galaxies in optical images, but were found to emit enormous amounts of radio waves. __Quasars__: Quasars are the most distant and most luminous type of AGN known; and their spectra doesn’t look like normal galaxies at all. __BL Lac objects__: Most AGN have strong emission lines, but a special class of AGN are notorious for having only very weak emission lines, if any at all. __Seyfert galaxies:__ are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen. __Optically Violent Variable Quasar (OVV quasar):__is a type of highly variable quasar. It is a subtype of blazar that consists of a few rare, bright radio galaxies, whose visible light output can change by 50% in a day. They are similar in appearance to BL Lacs but generally have a stronger broad emission line, and tend to have higher red shift components.
 * Types of AGN**

You can learn to differentiate different galaxies by their spectra here

Sources: [|AGN_Spectroscopy.pdf] KH & PF milestones: KH milestone and PF milestone edited by me http://en.wikipedia.org/wiki/Seyfert_galaxy http://en.wikipedia.org/wiki/OVV