Anactive galactic nucleus (AGN) is a compact region at the center of agalaxy that emits a significant amount of energy across theelectromagnetic spectrum, with characteristics indicating that this luminosity is not produced by thestars. Such excess, non-stellar emissions have been observed in theradio,microwave,infrared,optical,ultra-violet,X-ray, andgamma ray wavebands. A galaxy hosting an AGN is called anactive galaxy. The non-stellar radiation from an AGN is theorized to result from theaccretion of matter by asupermassive black hole at the center of its host galaxy. The super massive black hole at the center of ourMilky Way galaxy is not currently active but it is believed to have been active about 8 billion years ago.
Active galactic nuclei are the most luminous persistent sources ofelectromagnetic 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 onmodels 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 theaccretion disk, the degree ofobscuration of the nucleus bydust, and presence or absence ofjets. Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified asquasars. Ablazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced byrelativistic beaming.
During the first half of the 20th century, photographic observations of nearby galaxies detected some characteristic signatures of active galactic nucleus emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Some early observations included the first spectroscopic detection ofemission lines from the nuclei ofNGC 1068 andMessier 81 by Edward Fath (published in 1909),[1] and the discovery of thejet inMessier 87 byHeber Curtis (published in 1918).[2] Further spectroscopic studies by astronomers includingVesto Slipher,Milton Humason, andNicholas 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.[7] Galaxies observed as part of this study includedNGC 1068,NGC 4151,NGC 3516, andNGC 7469. Active galaxies such as these are known asSeyfert galaxies in honor of Seyfert's pioneering work.
The development ofradio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby activeelliptical galaxies such asMessier 87 andCentaurus A.[8] Another radio source,Cygnus A, was identified byWalter Baade andRudolph Minkowski as a tidally distorted galaxy with an unusualemission-line spectrum, having arecessional velocity of 16,700 kilometers per second.[9] The3C radio survey led to further progress in discovery of new radio sources as well as identifying thevisible-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 asquasi-stellar radio sources (later abbreviated as "quasars").
Soviet-Armenian astrophysicistViktor Ambartsumian introduced the concept of active galactic nuclei in the early 1950s.[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. 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."[11] His idea was initially received skeptically.[12][13]
A major breakthrough was the measurement of theredshift of the quasar3C 273 byMaarten Schmidt, published in 1963.[14] Schmidt noted that if this object wasextragalactic (outside theMilky 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 including3C 48, even more distant at redshift 0.37.[15]
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 asupermassive black hole was suggested as the source of quasars' power in papers byEdwin Salpeter andYakov Zeldovich in 1964.[16] In 1969Donald 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.[17] In the 1960s and 1970s, earlyX-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, bothobservational andtheoretical. 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 ofelectromagnetic 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 ongalaxy evolution.[citation needed]
Since the late 1960s it has been argued[18] that an AGN must be powered byaccretion of mass onto massive black holes (106 to 1010 times theSolar 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 highEddington luminosity. 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 thevelocity dispersion of the galactic bulge (theM–sigma relation) or with bulge luminosity.[19] Thus, AGN-like characteristics are expected whenever a supply of material for accretion comes within thesphere of influence of the central black hole.
In the standard model of AGN, cold material close to a black hole forms anaccretion 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, acorona of hot material forms above the accretion disc and caninverse-Compton scatterphotons 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 particularemission lines. A large fraction of the AGN's radiation may be obscured byinterstellar gas anddust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared.[citation needed]
Some accretion discs produce jets of twin, highlycollimated, 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, wherevery-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 thesynchrotron and theinverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation.[citation needed]
There exists a class of "radiatively inefficient" solutions to the equations that govern accretion. Several theories exist, but the most widely known of these is theAdvection Dominated Accretion Flow (ADAF).[20] In this type of accretion, which is important for accretion rates well below theEddington 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.[21] Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.
The observed AGNs are grouped into dozens of different sometimes overlapping classes.[22]AGNs are classified along multiple criteria. Two AGN may be in the same group according observations at one wavelength and in different groups according observations at another wavelength. This issues are believed to reflect the current early stages of understanding AGN. Classifications based on observations have not yet been interpreted with consistent physical models.[23]
One criteria is the radio-to-optical emission ratio or radio loudness parameter,where is the luminosity at the 5 GHz radio band and is the optical luminosity. TheAGN with and are called radio loud, otherwise they are radio quiet. This ratio is suspect in cases where the optical emission may be obscured by dust and direct star light along the line to the AGN. Alternatively this split can be defined as a radio luminosity cutoff at a fixed frequency, e.g.[23]
A second criteria is the existence of broad emission lines in the optical spectrum: type-1 has broad lines but type-2 does not.[24]
When the galaxy associated with an AGN is optically resolvable they are calledSeyfert galaxies. These are classed as type-1 or type-2 according to the existence of broad emission lines.[24]
Radio-quietquasars or QSOs. These are essentially more-luminous versions of Seyfert 1s. The distinction is arbitrary, and is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in optical images, because they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while other astronomers talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars, or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).
'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities, but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.[citation needed]
There are several subtypes of radio-loud active galactic nuclei.
Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
"Blazars" (BL Lacertae (BL Lac) objects andoptically violent variable (OVV) quasars) are distinguished by rapidly variable, polarized optical, radio, and X-ray emissions. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent, or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level.[25] OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet that is oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.
Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes.[26][27] 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.[28] Their optical and X-ray nuclear emission is consistent with originating purely in a jet.[29][30] 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 emission[31] 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.
Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions.A "strict" unification model proposes that the apparent differences between different types of objects arise simply because of their different orientations of the jet and obscuring torus as viewed on Earth.The obscuring torus, also called a "dusty torus" is a cool outer layer surrounding an accretion disk.[33] This model has had partial success, showing for example that Seyfert galaxies of type 1 and 2 are the same kinds of AGN viewed differently.[22]: 36 [34][32]
Other effects that might lead the same kind of astrophysical object to have distinctive observational characteristics include the accretion rate, the strength of the relativistic jet, obscuring effects of the galaxy surrounding the AGN, or time of observation relative to the formation of the AGN. AGNs are the subject of numerous on-going studies seeking to clarify the nature of AGNs.[22]: 37
It is expected that allsupermassive black holes at the center ofgalaxies have gone through high AGN activity to reach the mass we see today. These periods of high AGN activity can potentially affect theatmospheres ofplanets and theirhabitability. Planets located in compact galaxies such as “Red Nuggets” are likely to be more impacted than planets located in typicalelliptical galaxies such asM87 orspiral galaxies such as theMilky Way galaxy.For a planet with a high amount of initialoxygen in its atmosphere, AGN radiation may allow a thickerozone layer, possibly by shielding it from otherUV radiation. This would potentially increase the habitability of a planet.[35]
The supermassive black hole at the center of our galaxy (Sagittarius A*) experienced a phase of AGN activity 8 Gyr ago. This would have caused loss of atmospheres to planets within 1 kpc comparable to present-dayEarth. TheX-ray and extremeUV radiation also would have caused biological damage to surface life on planets without proper shielding. This would potentially hinder the development ofcomplex life within a few kiloparsecs.[36]
The inherentenergy from AGNs can also heat up the atmosphere of planets leading toatmospheric escape. The combined effect of AGN outflows would likely make all planets within 1kpc (~3,262light-years) of the center of a galaxy uninhabitable.[37]
3C 273: The first identified Quasar, notable for its relativistic jets (z=0.158).
3C 48: One of the earliest known Quasar with a measured redshift (z=0.367).
TON 618: An ultra-luminous quasar hosting one of the most massive known black holes (~66 billion solar masses), located at a redshift of z=2.219.
Twin Quasar: The first gravitationally lensed quasar, split into two images by an intervening galaxy (z=1.41).
Einstein Cross: A quasar lensed into four images forming a cross, demonstrating gravitational lensing predicted by general relativity (z=1.695).
Pōniuāʻena: One of the most distant quasars known (z=7.52), formed ~700 million years after the Big Bang, offering insights into early black hole formation.
CTA-102: A radio-loud quasar known for its variability (z=1.037).
Cloverleaf Quasar: A quasar lensed into four images resembling a cloverleaf, brightest known high-redshift source of CO emission (z=2.558).
Messier 87: A giant elliptical galaxy with a supermassive black hole (6.5 billion solar masses), imaged by the Event Horizon Telescope in 2019. Its jets extend thousands of light-years (z=0.00428).
Centaurus A: One of the closest radio galaxies, known for its dust lanes and relativistic jets (0.00183).
Cygnus A: A powerful radio source identified with a tidally distorted galaxy, a benchmark for studying AGN feedback (z=0.0561).
Hercules A: Features massive radio jets spanning ~1 million light-years, observed with radio telescopes like VLA (z=0.154).
Alcyoneus: The largest known radio galaxy, with jets extending 16.3 million light-years, discovered using LOFAR (z=0.2467).
TGSS J1530+1049: The most distant radio galaxy known (z=5.72), providing insights into early universe structures.
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^Baade, Walter; Minkowski, Rudolph (1954). "Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A.".The Astrophysical Journal.119: 206.Bibcode:1954ApJ...119..206B.doi:10.1086/145812.
^Komberg, B. V. (1992). "Quasars and Active Galactic Nuclei". InKardashev, N. S. (ed.).Astrophysics on the Threshold of the 21st Century.Taylor & Francis. p. 253.
^Laing, R. A.; Jenkins, C. R.; Wall, J. V.; Unger, S. W. (1994). "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.54: 201.Bibcode:1994ASPC...54..201L.
^Baum, S. A.; Zirbel, E. L.; O'Dea, Christopher P. (1995). "Toward Understanding the Fanaroff-Riley Dichotomy in Radio Source Morphology and Power".The Astrophysical Journal.451: 88.Bibcode:1995ApJ...451...88B.doi:10.1086/176202.
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