Editing Quasar (section)

{{Short description|Active galactic nucleus (AGN) containing a supermassive black hole}}
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{{About}}
{{redirect-distinguish|Quasi-stellar object|Quasi-star|Quaoar}}
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[[File:Artist's rendering ULAS J1120+0641.jpg|thumb|upright=1.35|Artist's rendering of the accretion disc in [[ULAS J1120+0641]], a very distant quasar containing a supermassive black hole with a mass two billion times that of the Sun<ref>{{cite news |title=Most Distant Quasar Found |url=http://www.eso.org/public/news/eso1122/ |access-date=4 July 2011 |newspaper=ESO Science Release}}</ref>]]
[[File:PKS 1127-145 X-rays.jpg|thumb|The [[Chandra X-ray Observatory|Chandra]] X-ray image is of the quasar [[PKS 1127-145]], a highly luminous source of X-rays and visible light about 10 billion light-years from Earth. An enormous X-ray jet extends at least a million light-years from the quasar. Image is 60 arcseconds on a side. [[Right ascension|RA]] 11h 30m 7.10s [[Declination|Dec]] −14° 49' 27" in [[Crater (constellation)|Crater]]. Observation date: May 28, 2000. Instrument: ACIS]]

A '''quasar''' ({{IPAc-en|ˈ|k|w|eɪ|z|ɑːr}} {{Respell|KWAY|zar}}) is an extremely [[Luminosity|luminous]] [[active galactic nucleus]] (AGN). It is sometimes known as a '''quasi-stellar object''', abbreviated '''QSO'''. The emission from an AGN is powered by accretion onto a [[supermassive black hole]] with a mass ranging from millions to tens of billions of [[solar mass]]es, surrounded by a gaseous [[Accretion disk|accretion disc]]. Gas in the disc falling towards the black hole heats up and releases [[energy]] in the form of [[electromagnetic radiation]]. The [[radiant energy]] of quasars is enormous; the most powerful quasars have [[luminosity|luminosities]] thousands of times greater than that of a [[galaxy]] such as the [[Milky Way]].<ref>{{cite journal |last1=Wu |first1=Xue-Bing |display-authors=etal |title=An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30 |journal=Nature |date=2015 |volume=518 |issue=7540 |pages=512–515 |doi=10.1038/nature14241 |pmid=25719667 |arxiv = 1502.07418 |bibcode = 2015Natur.518..512W |s2cid=4455954 }}</ref><ref>{{Cite book |last1=Frank |first1=Juhan |last2=King |first2=Andrew |last3=Raine |first3=Derek J. |title=Accretion Power in Astrophysics |edition=Third |bibcode=2002apa..book.....F |isbn=0521620538 |location=Cambridge, UK |publisher=Cambridge University Press |date=February 2002}}</ref> Quasars are usually categorized as a subclass of the more general category of AGN. The [[redshifts]] of quasars are of [[Expansion of the universe|cosmological origin]].<ref>{{Cite web |title=Quasars and Active Galactic Nuclei |url=https://ned.ipac.caltech.edu/level5/Kembhavi/Kem1_5.html |access-date=2020-08-31 |website=ned.ipac.caltech.edu}}</ref>

The term {{em|quasar}} originated as a [[Contraction (grammar)|contraction]] of "quasi-stellar ''[star-like]'' radio source"—because they were first identified during the 1950s as sources of radio-wave emission of unknown physical origin—and when identified in photographic images at visible wavelengths, they resembled faint, star-like points of light. High-resolution images of quasars, particularly from the [[Hubble Space Telescope]], have shown that quasars occur in the [[Central massive object|centers of galaxies]], and that some host galaxies are strongly [[Interacting galaxy|interacting]] or [[Galaxy merger|merging]] galaxies.<ref>{{cite journal |last1=Bahcall |first1=J. N. |display-authors=etal |title=Hubble Space Telescope Images of a Sample of 20 Nearby Luminous Quasars |journal=The Astrophysical Journal |date=1997 |volume=479 |issue=2 |pages=642–658 |doi=10.1086/303926 |arxiv = astro-ph/9611163 |bibcode = 1997ApJ...479..642B|s2cid=15318893 }}</ref> As with other categories of AGN, the observed properties of a quasar depend on many factors, including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disc relative to the observer, the presence or absence of a [[Astrophysical jet|jet]], and the degree of [[Extinction (astronomy)|obscuration]] by gas and [[Cosmic dust|dust]] within the host galaxy.

About a million quasars have been identified with reliable [[Astronomical spectroscopy|spectroscopic]] redshifts,<ref name="MILLIQUAS">{{cite web | title=Million Quasars Catalog, Version 8 (2 August 2023) | website=MILLIQUAS | date=2023-08-02 | url=https://heasarc.gsfc.nasa.gov/w3browse/all/milliquas.html | access-date=2023-11-20}}</ref> and between 2-3 million identified in [[Photometric_redshift|photometric]] catalogs.<ref name="Shu-2019">{{cite journal | last1=Shu | first1=Yiping | last2=Koposov | first2=Sergey E | last3=Evans | first3=N Wyn | last4=Belokurov | first4=Vasily | last5=McMahon | first5=Richard G | last6=Auger | first6=Matthew W | last7=Lemon | first7=Cameron A | title=Catalogues of active galactic nuclei from Gaia and unWISE data | journal=Monthly Notices of the Royal Astronomical Society | publisher=Oxford University Press (OUP) | volume=489 | issue=4 | date=2019-09-05 | issn=0035-8711 | doi=10.1093/mnras/stz2487 | pages=4741–4759| doi-access=free | arxiv=1909.02010 }}</ref><ref name="Quaia-2023">{{cite journal | last1=Storey-Fisher | first1=Kate | last2=Hogg | first2=David W. | last3=Rix | first3=Hans-Walter | last4=Eilers | first4=Anna-Christina | last5=Fabbian | first5=Giulio | last6=Blanton | first6=Michael | last7=Alonso | first7=David | year=2024 | title=Quaia, the Gaia-unWISE Quasar Catalog: An All-Sky Spectroscopic Quasar Sample | journal=AAS Journals | volume=964 | issue=1 | page=69 | doi=10.3847/1538-4357/ad1328 | doi-access=free | arxiv=2306.17749 | bibcode=2024ApJ...964...69S }}</ref> The [[Markarian 231|nearest known]] quasar is about 600 million [[light-year]]s from Earth, while the record for the [[UHZ1|most distant known AGN]] is at a redshift of 10.1, corresponding to a [[Comoving and proper distances|comoving distance]] of 31.6 billion light-years, or a look-back time of 13.2 billion years.<ref>{{Cite web |title=APOD: 2023 November 10 - UHZ1: Distant Galaxy and Black Hole |url=https://apod.nasa.gov/apod/ap231110.html |access-date=2023-11-10 |website=apod.nasa.gov}}</ref><ref name=":0">{{Cite journal |last1=Whalen |first1=Daniel J. |last2=Latif |first2=Muhammad A. |last3=Mezcua |first3=Mar |date=2023-10-01 |title=Radio Emission From a z = 10.1 Black Hole in UHZ1 |journal=The Astrophysical Journal |volume=956 |issue=2 |pages=133 |doi=10.3847/1538-4357/acf92c |issn=0004-637X|doi-access=free |arxiv=2308.03837 |bibcode=2023ApJ...956..133W }}</ref>

Quasar discovery surveys have shown that quasar activity was more common in the distant past; the peak epoch was approximately 10&nbsp;billion years ago.<ref>{{cite journal |last1=Schmidt |first1=Maarten |author-link=Maarten Schmidt |last2=Schneider |first2=Donald |last3=Gunn |first3=James |date=1995 |title=Spectroscopic CCD Surveys for Quasars at Large Redshift. IV.&nbsp;Evolution of the Luminosity Function from Quasars Detected by Their Lyman-Alpha Emission |journal=The Astronomical Journal |volume=110 |page=68 |bibcode=1995AJ....110...68S |doi=10.1086/117497}}</ref> Concentrations of multiple quasars are known as [[large quasar group]]s and may constitute some of the [[galaxy filament|largest known structures]] in the universe if the observed groups are good tracers of mass distribution.