Editing Active galactic nucleus (section)

== History ==
[[File:Best image of bright quasar 3C 273 (10953173335).jpg|thumb|upright=1.2|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 spikes]] caused by the telescope optics.]]
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 of [[emission lines]] from the nuclei of [[NGC 1068]] and [[Messier 81]] by Edward Fath (published in 1909),<ref>{{cite journal |last1=Fath |first1=Edward A. |date=1909 |title=The spectra of some spiral nebulae and globular star clusters |journal=Lick Observatory Bulletin |volume=5 |page=71 |bibcode=1909LicOB...5...71F |doi=10.5479/ADS/bib/1909LicOB.5.71F |hdl-access=free |hdl=2027/uc1.c2914873}}</ref> and the discovery of the [[Astrophysical jet|jet]] in [[Messier 87]] by [[Heber Curtis]] (published in 1918).<ref>{{cite journal |last1=Curtis |first1=Heber D. |date=1918 |title=Descriptions of 762 Nebulae and Clusters Photographed with the Crossley Reflector |journal=Publications of Lick Observatory |volume=13 |page=9 |bibcode=1918PLicO..13....9C}}</ref> Further spectroscopic studies by astronomers including [[Vesto Slipher]], [[Milton Humason]], and [[Nicholas Mayall]] noted the presence of unusual emission lines in some galaxy nuclei.<ref>{{cite journal |last1=Slipher |first1=Vesto |date=1917 |title=The spectrum and velocity of the nebula N.G.C. 1068 (M 77) |journal=Lowell Observatory Bulletin |volume=3 |page=59 |bibcode=1917LowOB...3...59S}}</ref><ref>{{cite journal |last1=Humason |first1=Milton L. |date=1932 |title=The Emission Spectrum of the Extra-Galactic Nebula N. G. C. 1275 |journal=Publications of the Astronomical Society of the Pacific |volume=44 |issue=260 |page=267 |bibcode=1932PASP...44..267H |doi=10.1086/124242 |doi-access=free}}</ref><ref>{{cite journal |last1=Mayall |first1=Nicholas U. |date=1934 |title=The Spectrum of the Spiral Nebula NGC 4151 |journal=Publications of the Astronomical Society of the Pacific |volume=46 |issue=271 |page=134 |bibcode=1934PASP...46..134M |doi=10.1086/124429 |s2cid=119741164 |doi-access=free}}</ref><ref>{{cite journal |last1=Mayall |first1=Nicholas U. |date=1939 |title=The occurrence of λ3727 [O II] in the spectra of extragalactic nebulae |journal=Lick Observatory Bulletin |volume=19 |page=33 |bibcode=1939LicOB..19...33M |doi=10.5479/ADS/bib/1939LicOB.19.33M |doi-access=free}}</ref> 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.<ref>{{cite journal |last1=Seyfert |first1=Carl K. |date=1943 |title=Nuclear Emission in Spiral Nebulae |journal=The Astrophysical Journal |volume=97 |page=28 |bibcode=1943ApJ....97...28S |doi=10.1086/144488}}</ref> 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]].<ref>{{cite journal|last1=Bolton|first1=J. G.|last2=Stanley|first2=G. J.|last3=Slee|first3=O. B.|title=Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation|journal=Nature|date=1949|volume=164|issue=4159|page=101|doi=10.1038/164101b0|bibcode=1949Natur.164..101B|s2cid=4073162|doi-access=free}}</ref> 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.<ref>{{cite journal |last1=Baade |first1=Walter |last2=Minkowski |first2=Rudolph |date=1954 |title=Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A. |journal=The Astrophysical Journal |volume=119 |page=206 |bibcode=1954ApJ...119..206B |doi=10.1086/145812}}</ref> The [[Third Cambridge Catalogue of Radio Sources|3C radio survey]] led to further progress in discovery of new radio sources as well as identifying the [[Visible light|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 [[Quasar|quasi-stellar radio sources]] (later abbreviated as "quasars").

Soviet Armenian astrophysicist [[Viktor Ambartsumian]] introduced Active Galactic Nuclei in the early 1950s.<ref>{{cite journal |last1=Israelian |first1=Garik |author-link=Garik Israelian |date=1997 |title=Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908]–1996 |url=http://aas.org/obituaries/victor-amazasp-ambartsumian-1908-1996?qt-obituary_sort=0 |journal=[[Bulletin of the American Astronomical Society]] |volume=29 |issue=4 |pages=[http://adsabs.harvard.edu/full/1997BAAS...29.1466I 1466–1467] |archive-url=https://web.archive.org/web/20150911212352/http://aas.org/obituaries/victor-amazasp-ambartsumian-1908-1996?qt-obituary_sort=0 |archive-date=2015-09-11}}</ref> 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."<ref>{{cite web |last1=McCutcheon |first1=Robert A. |author-link=Robyn McCutcheon |title=Ambartsumian, Viktor Amazaspovich |url=https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/ambartsumian-viktor-amazaspovich |publisher=[[Encyclopedia.com]] |work=[[Complete Dictionary of Scientific Biography]] |archive-url=https://web.archive.org/web/20191203145604/https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/ambartsumian-viktor-amazaspovich |archive-date=3 December 2019 |date=1 November 2019}}</ref> His idea was initially accepted skeptically.<ref>{{cite journal |last1=Petrosian |first1=Artashes R. |last2=Harutyunian |first2=Haik A. |last3=Mickaelian |first3=Areg M. |title=Victor Amazasp Ambartsumian |journal=[[Physics Today]] |date=June 1997 |volume=50 |issue=6 |page=106 |doi=10.1063/1.881754 |doi-access=free }} ([https://physicstoday.scitation.org/doi/pdf/10.1063/1.881754 PDF])</ref><ref>{{cite book |last1=Komberg |first1=B. V. |editor1-last=Kardashev |editor1-first=N. S. |editor1-link=Nikolai Kardashev |title=Astrophysics on the Threshold of the 21st Century |date=1992 |publisher=[[Taylor & Francis]] |page=[https://books.google.com/books?id=QTfXSeOzrI4C&dq=ambartsumian+1958+%22first%22+active+galactic&pg=PA253 253] |chapter=Quasars and Active Galactic Nuclei}}</ref>

A major breakthrough was the measurement of the [[redshift]] of the quasar [[3C 273]] by [[Maarten Schmidt]], published in 1963.<ref>{{cite journal |last1=Schmidt |first1=Maarten |date=1963 |title=3C 273 : A Star-Like Object with Large Red-Shift |journal=Nature |volume=197 |issue=4872 |page=1040 |bibcode=1963Natur.197.1040S |doi=10.1038/1971040a0 |s2cid=4186361 |doi-access=free}}</ref> 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.<ref>{{cite journal|last1=Greenstein|first1=J. L.|last2=Matthews|first2=T. A.|title=Red-Shift of the Unusual Radio Source: 3C 48|journal=Nature|date=1963|volume=197|issue=4872|page=1041|doi=10.1038/1971041a0|bibcode = 1963Natur.197.1041G |s2cid=4193798}}</ref>

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.<ref>{{cite journal|last1=Shields|first1=G. A.|title=A Brief History of Active Galactic Nuclei|journal=Publications of the Astronomical Society of the Pacific|date=1999|volume=111|issue=760|page=661|doi=10.1086/316378|bibcode=1999PASP..111..661S|arxiv = astro-ph/9903401 |s2cid=18953602}}</ref> 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.<ref>{{cite journal|last1=Lynden-Bell|first1=Donald|title=Galactic Nuclei as Collapsed Old Quasars|journal=Nature|date=1969|volume=223|issue=5207|page=690|doi=10.1038/223690a0|bibcode=1969Natur.223..690L|s2cid=4164497}}</ref> 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 astronomy|observational]] and [[Theoretical astrophysics|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]].