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active galactic nucleus
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{{short description|Compact region at a galaxy's center with abnormally high luminosity}}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 the luminosity is not produced by stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. 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.The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and presence or absence of jets.Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as quasars. A blazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by relativistic beaming.- the content below is remote from Wikipedia
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History
File:Best image of bright quasar 3C 273.jpg|thumb|Quasar 3C 273 observed by the Hubble Space Telescope. The relativistic jet of 3C 273 appears to the left of the bright quasar, and the four straight lines pointing outward from the central source are diffraction spikesdiffraction spikesDuring the first half of the 20th century, photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Some early observations included the first spectroscopic detection of emission lines from the nuclei of NGC 1068 and Messier 81 by Edward Fath (published in 1909),JOURNAL, Fath, E. A., The spectra of some spiral nebulae and globular star clusters, Lick Observatory Bulletin, 1909, 5, 71, 10.5479/ADS/bib/1909LicOB.5.71F, 1909LicOB...5...71F, 2027/uc1.c2914873, free, and the discovery of the jet in Messier 87 by Heber Curtis (published in 1918).JOURNAL, Curtis, H. D., Descriptions of 762 Nebulae and Clusters Photographed with the Crossley Reflector, Publications of Lick Observatory, 1918, 13, 9, 1918PLicO..13....9C, Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, and Nicholas Mayall noted the presence of unusual emission lines in some galaxy nuclei.JOURNAL, Slipher, V., The spectrum and velocity of the nebula N.G.C. 1068 (M 77), Lowell Observatory Bulletin, 1917, 3, 59, 1917LowOB...3...59S, JOURNAL, Humason, M. L., The Emission Spectrum of the Extra-Galactic Nebula N. G. C. 1275, Publications of the Astronomical Society of the Pacific, 1932, 44, 260, 267, 10.1086/124242, 1932PASP...44..267H, free, JOURNAL, Mayall, N. U., The Spectrum of the Spiral Nebula NGC 4151, Publications of the Astronomical Society of the Pacific, 1934, 46, 271, 134, 10.1086/124429, 1934PASP...46..134M, 119741164, free, JOURNAL, Mayall, N. U., The occurrence of λ3727 [O II] in the spectra of extragalactic nebulae, Lick Observatory Bulletin, 1939, 19, 33, 10.5479/ADS/bib/1939LicOB.19.33M, 1939LicOB..19...33M, free, 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.JOURNAL, Seyfert, C. K., Nuclear Emission in Spiral Nebulae, The Astrophysical Journal, 1943, 97, 28, 10.1086/144488, 1943ApJ....97...28S, Galaxies observed as part of this study included NGC 1068, NGC 4151, NGC 3516, and NGC 7469. Active galaxies such as these are known as Seyfert galaxies in honor of Seyfert's pioneering work.The development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87 and Centaurus A.JOURNAL, Bolton, J. G., Stanley, G. J., Slee, O. B., Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation, Nature, 1949, 164, 4159, 101, 10.1038/164101b0, 1949Natur.164..101B, 4073162, free, 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.JOURNAL, Baade, W., Minkowski, R., Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A., The Astrophysical Journal, 1954, 119, 206, 10.1086/145812, 1954ApJ...119..206B, The 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, and were classified as quasi-stellar radio sources (later abbreviated as "quasars").Soviet Armenian astrophysicist Viktor Ambartsumian introduced Active Galactic Nuclei in the early 1950s.JOURNAL, Israelian, Garik, Garik Israelian, Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908] -1996,weblink Bulletin of the American Astronomical Society, 1997, 29, 4, 1466â1467,weblink" title="web.archive.org/web/20150911212352weblink">weblink 2015-09-11, 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. For these explosions to occur, galactic nuclei must contain bodies of huge mass and unknown nature. From this point forward Active Galactic Nuclei (AGN) became a key component in theories of galactic evolution."WEB, McCutcheon, Robert A., Robyn McCutcheon, Ambartsumian, Viktor Amazaspovich,weblink Encyclopedia.com, Complete Dictionary of Scientific Biography,weblink 3 December 2019, 1 November 2019, His idea was initially accepted skeptically.JOURNAL, Petrosian, Artashes R., Harutyunian, Haik A., Mickaelian, Areg M., Victor Amazasp Ambartsumian, Physics Today, June 1997, 50, 6, 106, 10.1063/1.881754, free, (PDF)BOOK, Komberg, B. V., Kardashev, N. S., Nikolai Kardashev, Astrophysics on the Threshold of the 21st Century, 1992, Taylor & Francis, 253, Quasars and Active Galactic Nuclei, A major breakthrough was the measurement of the redshift of the quasar 3C 273 by Maarten Schmidt, published in 1963.JOURNAL, Schmidt, M., 3C 273 : A Star-Like Object with Large Red-Shift, Nature, 1963, 197, 4872, 1040, 10.1038/1971040a0, 1963Natur.197.1040S, 4186361, free, Schmidt noted that if this object was extragalactic (outside the Milky Way, at a cosmological distance) then its large redshift of 0.158 implied that it was the nuclear region of a galaxy about 100 times more powerful than other radio galaxies that had been identified. Shortly afterward, optical spectra were used to measure the redshifts of a growing number of quasars including 3C 48, even more distant at redshift 0.37.JOURNAL, Greenstein, J. L., Matthews, T. A., Red-Shift of the Unusual Radio Source: 3C 48, Nature, 1963, 197, 4872, 1041, 10.1038/1971041a0, 1963Natur.197.1041G, 4193798, The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars. 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.JOURNAL, Shields, G. A., A Brief History of Active Galactic Nuclei, Publications of the Astronomical Society of the Pacific, 1999, 111, 760, 661, 10.1086/316378, 1999PASP..111..661S, astro-ph/9903401, 18953602, In 1969 Donald Lynden-Bell proposed that nearby galaxies contain supermassive black holes at their centers as relics of "dead" quasars, and that black hole accretion was the power source for the non-stellar emission in nearby Seyfert galaxies.JOURNAL, Lynden-Bell, Donald, Galactic Nuclei as Collapsed Old Quasars, Nature, 1969, 223, 5207, 690, 10.1038/223690a0, 1969Natur.223..690L, 4164497, In the 1960s and 1970s, early X-ray astronomy observations demonstrated that Seyfert galaxies and quasars are powerful sources of X-ray emission, which originates from the inner regions of black hole accretion disks.Today, AGN are a major topic of astrophysical research, both observational and theoretical. AGN research encompasses observational surveys to find AGN over broad ranges of luminosity and redshift, examination of the cosmic evolution and growth of black holes, studies of the physics of black hole accretion and the emission of electromagnetic radiation from AGN, examination of the properties of jets and outflows of matter from AGN, and the impact of black hole accretion and quasar activity on galaxy evolution.Models
File:Lasers and supermassive black holes UGC 6093.jpg|thumb|UGC 6093UGC 6093For a long time it has been arguedJOURNAL, 10.1038/223690a0, 223, 5207, 690â694, Lynden-Bell, D., Galactic Nuclei as Collapsed Old Quasars, Nature, 1969, 1969Natur.223..690L, 4164497, that an AGN must be powered by accretion of mass onto massive black holes (106 to 1010 times the Solar mass). AGN are both compact and persistently extremely luminous. Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity, and as a result, it can provide the observed high persistent luminosity. Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the Mâsigma relation) or with bulge luminosity.JOURNAL, 589, 1, L21âL24, Marconi, A., L. K. Hunt, The Relation between Black Hole Mass, Bulge Mass, and Near-Infrared Luminosity, The Astrophysical Journal, 2003, 10.1086/375804, 2003ApJ...589L..21M s2cid = 15911138, Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.Accretion discIn the standard model of AGN, cold material close to a black hole forms an accretion disc. Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The expected spectrum of an accretion disc peaks in the optical-ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and can inverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomic material close to the black hole and this in turn radiates at particular emission lines. 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.Relativistic jetsFile:M87 jet.jpg|right|thumb| 200px| Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. The blue synchrotron radiationsynchrotron radiationSome accretion discs produce jets of twin, highly collimated, and fast outflows that emerge in opposite directions from close to the disc. The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low. 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. However, they radiate in all wavebands from the radio through to the gamma-ray range via the synchrotron and the inverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation.Radiatively inefficient AGNThere exists a class of "radiatively inefficient" solutions to the equations that govern accretion. Several theories exist, but the most widely known of these is the Advection Dominated Accretion Flow (ADAF).JOURNAL, 428, L13, Narayan, R., I. Yi, Advection-Dominated Accretion: A Self-Similar Solution, Astrophys. J., 1994 | bibcode = 1994ApJ...428L..13N | s2cid = 8998323, In this type of accretion, which is important for accretion rates well below the Eddington limit, the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole. Radiatively inefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities.JOURNAL, 277, 2, L55âL58, Fabian, A. C., M. J. Rees, The accretion luminosity of a massive black hole in an elliptical galaxy, Monthly Notices of the Royal Astronomical Society, 1995 | bibcode = 1995MNRAS.277L..55F | s2cid = 18890265, Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.Particle accelerationAGN are a candidate source of high and ultra-high energy cosmic rays (see also Centrifugal mechanism of acceleration).Observational characteristicsAmong the many interesting characteristics of AGNs:JOURNAL, Padovani, P., Alexander, D. M., Assef, R. J., De Marco, B., Giommi, P., Hickox, R. C., Richards, G. T., SmolÄiÄ, V., Hatziminaoglou, E., Mainieri, V., Salvato, M., November 2017, Active galactic nuclei: what's in a name?,weblink The Astronomy and Astrophysics Review, en, 25, 1, 10.1007/s00159-017-0102-9, 0935-4956, 1707.07134,
Types of active galaxyIt is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. Radio-loud objects have emission contributions from both the jet(s) and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths.AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences.Radio-quiet AGN
Radio-loud AGNSee main article Radio galaxy for a discussion of the large-scale behaviour of the jets. Here, only the active nuclei are discussed.
| author3=J. V. Wall, S. W. Unger, Spectrophotometry of a Complete Sample of 3CR Radio Sources: Implications for Unified Models, The First Stromlo Symposium: The Physics of Active Galaxies. ASP Conference Series, 1994, 1994ASPC...54..201L, Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism.JOURNAL, 451, 88, Baum, S. A., E. L., Zirbel, Toward Understanding the Fanaroff-Riley Dichotomy in Radio Source Morphology and Power, The Astrophysical Journal, 1995, 10.1086/176202, 1995ApJ...451...88B, O'Dea, Christopher P., Their optical and X-ray nuclear emission is consistent with originating purely in a jet.JOURNAL, 394, 791â800, Chiaberge, M. | author3=A. Celotti, Understanding the nature of FRII optical nuclei: a new diagnostic plane for radio galaxies, Astron. Astrophys., 2002, 10.1051/0004-6361:20021204, 2002A&A...394..791C, astro-ph/0207654 | s2cid = 4308057, JOURNAL, 10.1111/j.1365-2966.2006.10615.x, 370, 4, 1893â1904, Hardcastle, M. J. | author3=J. H. Croston, The X-ray nuclei of intermediate-redshift radio sources, Monthly Notices of the Royal Astronomical Society, 2006, 2006MNRAS.370.1893H | s2cid = 14632376, They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emissionJOURNAL, 220, Part 1, Grandi, S. A., D. E. Osterbrock, Optical spectra of radio galaxies, Astrophysical Journal, 1978, 10.1086/155966, 1978ApJ...220..783G, 783, probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.{| class="wikitable" style="margin:auto;"
|+ Features of different types of galaxies
nuclei
! rowspan=2 | Galaxy type ! rowspan=2 | Active
! colspan="2" |Emission lines
radio
! rowspan=2 | X-rays ! colspan="2" |Excess of ! rowspan=2 |Strong
! rowspan=2 |Jets
loud
! rowspan=2 |Variable ! rowspan=2 |Radio
|-
!Narrow !! Broad !! UV !! Far-IR |- !Normal (non-AGN) | no || weak || no || weak || no || no || no || no || no || no |- !LINER | unknown || weak || weak || weak || no || no || no || no || no || no |- !Seyfert I | yes || yes || yes || some || some || yes || few || no || yes || no |- !Seyfert II | yes || yes || no || some || some || yes || few || no || yes || no |- !Quasar | yes || yes || yes || some || yes || yes || some || some || yes || some |- !Blazar | yes || no || some || yes || yes || no ||yes || yes || yes ||yes |- !BL Lac | yes || no || no/faint || yes || yes || no || yes || yes || yes || yes |- !OVV | yes || no || stronger than BL Lac|| yes || yes || no || yes || yes || yes || yes |- !Radio galaxy | yes || some || some || some || some || yes || yes || yes || yes || yes |- |
Unification of AGN species
(File:Emmaalexander unified agn.png|thumb|350px|right|Unified AGN models)Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer.JOURNAL, 31, 1, 473â521, Antonucci, R., Unified Models for Active Galactic Nuclei and Quasars, Annual Review of Astronomy and Astrophysics, 1993, 10.1146/annurev.aa.31.090193.002353, 1993ARA&A..31..473A, JOURNAL, 107, 803â845, Urry, P., Paolo Padovani, Unified schemes for radioloud AGN, Publications of the Astronomical Society of the Pacific, 1995, 10.1086/133630, 1995PASP..107..803URadio-quiet unification
At low luminosities, the objects to be unified are Seyfert galaxies. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. 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. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission linesâwhich are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of the nuclei of Seyfert 2s also support this picture.At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.Radio-loud unification
Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. 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). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true.JOURNAL, 331, 6152, 149â151, Laing, R. A., The sidedness of jets and depolarization in powerful extragalactic radio sources, Nature, 1988, 10.1038/331149a0Criticism of the radio-quiet unification
In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of the key predictions of the Unified Model, e.g. that each Seyfert 2 has an obscured Seyfert 1 nucleus (a hidden broad-line region).Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e.g. intense, nuclear starbursts. Spectropolarimetric studiesJOURNAL, 554, 1, L19âL23, Tran, H.D., Hidden Broad-Line Seyfert 2 Galaxies in the CFA and 12 $mu$M Samples, The Astrophysical Journal, 2001Cosmological uses and evolution
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.Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests that massive black holes formed early on and that the conditions for the formation of luminous AGN were more common in the early universe, such as a much higher availability of cold gas near the centre of galaxies than at present. It also implies that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well understood due to the difficulty of observing these objects at high redshifts.See also
- {{annotated link|Mâsigma relation}}
- {{annotated link|Quasar}}
- {{annotated link|Radio galaxy}}
- {{annotated link|Astrophysical jetRelativistic jet|Relativistic jet}}
- {{annotated link|Supermassive black hole}}
- {{annotated link|Reverberation mapping}}
References
{{Reflist|30em}}External links
- {{Commons category-inline|Active galactic nuclei}}
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