Active galactic nucleus
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars.
Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands.
The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe and, as such, can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.
[2] Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, and Nicholas Mayall noted the presence of unusual emission lines in some galaxy nuclei.
[3][4][5][6] In 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines.
[8] Another radio source, Cygnus A, was identified by Walter Baade and Rudolph Minkowski as a tidally distorted galaxy with an unusual emission-line spectrum, having a recessional velocity of 16,700 kilometers per second.
[10] At the Solvay Conference on Physics in 1958, Ambartsumian presented a report arguing that "explosions in galactic nuclei cause large amounts of mass to be expelled.
Accretion of gas onto a supermassive black hole was suggested as the source of quasars' power in papers by Edwin Salpeter and Yakov Zeldovich in 1964.
[19] Thus, AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.
A large fraction of the AGN's radiation may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared.
The jets have their most obvious observational effects in the radio waveband, where very-long-baseline interferometry can be used to study the synchrotron radiation they emit at resolutions of sub-parsec scales.
In Seyfert 2s the nucleus is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or (soft) X-ray emission.
These can be unified with narrow-line radio galaxies in a manner directly analogous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission).
[33][34][35] X-ray evidence, where available, supports the unified picture: radio galaxies show evidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission from the sources' large-scale hot-gas environment.
[46] While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory.
Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN[52] have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history.
While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete.
[57] In this model, the relative accretion rate (termed the "Eddington ratio") of the black hole has a significant impact on the observed features of the AGN.
Black Holes with higher Eddington ratios appear to be more likely to be unobscured, having cleared away locally obscuring material in a very short timescale.
For a long time, active galaxies held all the records for the highest-redshift objects known either in the optical or the radio spectrum, because of their high luminosity.
They still have a role to play in studies of the early universe, but it is now recognised that an AGN gives a highly biased picture of the "typical" high-redshift galaxy.
