Editing Quasar (section)

== Properties ==
More than {{val|900000|fmt=commas}} quasars have been found (as of July 2023),<ref name="MILLIQUAS"/> most from the [[Sloan Digital Sky Survey]]. All observed quasar spectra have redshifts between 0.056 and 10.1 (as of 2024), which means they range between 600 million and 30&nbsp;billion [[comoving distance|light-years away from Earth]]. Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.

The power of quasars originates from supermassive black holes that are believed to exist at the core of most galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are revolving around tremendous masses with very steep gravity gradients, suggesting black holes.

Although quasars appear faint when viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe. The brightest quasar in the sky is [[3C 273]] in the [[constellation]] of [[Virgo (constellation)|Virgo]]. It has an average [[apparent magnitude]] of 12.8 (bright enough to be seen through a medium-size amateur [[telescope]]), but it has an [[absolute magnitude]] of −26.7.<ref name="3C 273 article">{{cite journal |title=The Quasi-Stellar Radio Sources 3C 48 and 3C 273 |bibcode=1964ApJ...140....1G |journal=The Astrophysical Journal |doi=10.1086/147889 |volume=140 |pages=1 |year=1964 |last1=Greenstein |first1=Jesse L. |last2=Schmidt |first2=Maarten|s2cid=123147304 |doi-access=free }}<!--|access-date=25 April 2014--></ref> From a distance of about 33&nbsp;light-years, this object would shine in the sky about as brightly as the [[Sun]]. This quasar's [[luminosity]] is, therefore, about 4&nbsp;trillion (4{{e|12}}) times that of the Sun, or about 100 times that of the total light of giant galaxies like the [[Milky Way]].<ref name="3C 273 article"/> This assumes that the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet. In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth. Such quasars are called [[blazars]].

The hyperluminous quasar [[APM&nbsp;08279+5255]] was, when discovered in 1998, given an [[absolute magnitude]] of −32.2. High-resolution imaging with the [[Hubble Space Telescope]] and the 10&nbsp;m [[Keck Telescope]] revealed that this system is [[gravitational lensing|gravitationally lensed]]. A study of the gravitational lensing of this system suggests that the light emitted has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.

Quasars were much more common in the early universe than they are today. This discovery by [[Maarten Schmidt]] in 1967 was early strong evidence against [[Steady-state theory|steady-state cosmology]] and in favor of the [[Big Bang]] cosmology. Quasars show the locations where supermassive black holes are growing rapidly (by [[Accretion (astrophysics)|accretion]]). Detailed simulations reported in 2021 showed that galaxy structures, such as spiral arms, use gravitational forces to 'put the brakes on' gas that would otherwise orbit galaxy centers forever; instead the braking mechanism enabled the gas to fall into the supermassive black holes, releasing enormous radiant energies.<ref>{{cite web |title=New simulation shows how galaxies feed their supermassive black holes |url=https://www.sciencedaily.com/releases/2021/08/210817131435.htm |website=sciencedaily.com |access-date=31 August 2021 |date=17 August 2021 |quote=First model to show how gas flows across universe into a supermassive black hole’s center.}}</ref><ref>{{Cite journal |last=Anglés-Alcázar |first=Daniel |display-authors=etal |date=August 2021 |title=Cosmological Simulations of Quasar Fueling to Subparsec Scales Using Lagrangian Hyper-refinement |journal=The Astrophysical Journal |volume=917 |issue=2 |page=53 |arxiv=2008.12303 |bibcode=2021ApJ...917...53A |doi=10.3847/1538-4357/ac09e8 |issn=0004-637X |s2cid=221370537 |doi-access=free}}</ref> These black holes co-evolve with the mass of stars in their host galaxy in a way not fully understood at present. One idea is that jets, radiation and [[Disk wind|winds]] created by the quasars shut down the formation of new stars in the host galaxy, a process called "feedback". The jets that produce strong radio emission in some quasars at the centers of [[clusters of galaxies]] are known to have enough power to prevent the hot gas in those clusters from cooling and falling on to the central galaxy.

Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale as to allow the coordination of the luminosity variations. This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars. The conversion of [[gravitational potential energy]] to radiation by infalling to a black hole converts between 6% and 32% of the mass to energy, compared to 0.7% for the conversion of mass to energy in a star like the Sun.<ref name="Lambourne" /> It is the only process known that can produce such high power over a very long term. (Stellar explosions such as [[supernova]]s and [[gamma-ray burst]]s, and direct [[matter]]–[[antimatter]] annihilation, can also produce very high power output, but supernovae only last for days, and the universe does not appear to have had large amounts of antimatter at the relevant times.)

Since quasars exhibit all the properties common to other [[active galaxy|active galaxies]] such as [[Seyfert galaxy|Seyfert galaxies]], the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes. To create a luminosity of 10<sup>40</sup>&nbsp;[[watt]]s (the typical brightness of a quasar), a supermassive black hole would have to consume the material equivalent of 10 solar masses per year. The brightest known quasars devour 1,000 solar masses of material every year (equivalent to 10 Earths per second). Quasar luminosities can vary considerably over time, depending on their surroundings. Since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.

Radiation from quasars is partially "nonthermal" (i.e., not due to [[black-body radiation]]), and approximately 10% are observed to also have jets and lobes like those of [[radio galaxy|radio galaxies]] that also carry significant (but poorly understood) amounts of energy in the form of particles moving at [[relativistic speed]]s. Extremely high energies might be explained by several mechanisms (see [[Fermi acceleration]] and [[Centrifugal mechanism of acceleration]]). Quasars can be detected over the entire observable [[electromagnetic spectrum]], including [[radio waves|radio]], [[infrared]], [[visible light]], [[ultraviolet]], [[X-ray]] and even [[gamma ray]]s. Most quasars are brightest in their rest-frame ultraviolet [[wavelength]] of 121.6&nbsp;[[nanometer|nm]] [[Lyman series|Lyman-alpha]] emission line of hydrogen, but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 900.0&nbsp;nm, in the near infrared. A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light. When viewed downward, these appear as [[blazar]]s and often have regions that seem to move away from the center faster than the speed of light ([[superluminal]] expansion). This is an optical illusion due to the properties of [[special relativity]].

Quasar redshifts are measured from the strong [[spectral line]]s that dominate their visible and ultraviolet emission spectra. These lines are brighter than the continuous spectrum. They exhibit [[Doppler broadening]] corresponding to mean speed of several percent of the speed of light. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the [[Lyman series]] and [[Balmer series]]), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization.

Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by [[inverse Compton scattering]] of lower-energy photons by the radio-emitting electrons in the jet.<ref name="Dooling">{{Cite web |last=Dooling |first=Dave |date=18 November 1999 |title=BATSE finds most distant quasar yet seen in soft gamma rays Discovery will provide insight on formation of galaxies |url=https://science.nasa.gov/NEWHOME/HEADLINES/ast24nov99_1.htm |url-status=dead |archive-url=https://web.archive.org/web/20090723104615/http://science.nasa.gov/newhome/headlines/ast24nov99_1.htm |archive-date=2009-07-23 |website=NASA Science}}</ref>

{{anchor|iron quasar}}''Iron quasars'' show strong emission lines resulting from low-ionization [[iron]] (Fe&nbsp;{{abbr|II|singly ionized}}), such as IRAS&nbsp;18508-7815.

<gallery widths="200" heights="200">
File:Bright halos around distant quasars.jpg|Bright halos around 18 distant quasars<ref>{{cite web |title=Bright halos around distant quasars |url=http://www.eso.org/public/images/eso1638a/ |access-date=26 October 2016 |website=eso.org}}</ref>
File:PKS 1127-145 X-rays.jpg|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. Observation date: May 28, 2000. Instrument: ACIS
File:Quasar HE 1104-1805.jpg|Gravitationally lensed quasar HE 1104-1805<ref>{{cite news |title=Gravitationally lensed quasar HE 1104-1805 |url=http://www.spacetelescope.org/images/heic1116a/ |access-date=4 November 2011 |newspaper=ESA/Hubble Press Release}}</ref>
File:Artist's impression of mysterious alignment of quasar rotation axes.ogv|Animation shows the alignments between the spin axes of quasars and the large-scale structures that they inhabit.
</gallery>

=== Spectral lines, reionization, and the early universe===
{{main|Reionization|Chronology of the Universe}}
[[File:Fingerprint of the early Universe.jpg|thumb|Spectrum from quasar HE&nbsp;0940-1050 after it has travelled through [[intergalactic medium]]]]
[[File:QuasarStarburst.jpg|thumb|right|This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous [[starburst (astronomy)|starburst]] ever seen in such a combination.]]

Quasars also provide some clues as to the end of the [[Big Bang]]'s [[reionization]]. The oldest known quasars ([[redshift|''z'']]&nbsp;=&nbsp;6){{needs update|date=January 2021}} display a [[Gunn–Peterson trough]] and have absorption regions in front of them indicating that the [[intergalactic medium]] at that time was [[neutral gas]]. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the [[Lyman-alpha forest]]; this indicates that the intergalactic medium has undergone reionization into [[plasma (physics)|plasma]], and that neutral gas exists only in small clouds.

The intense production of [[ionization|ionizing]] [[ultraviolet]] radiation is also significant, as it would provide a mechanism for reionization to occur as galaxies form. Despite this, current theories suggest that quasars were not the primary source of reionization; the primary causes of reionization were probably the earliest generations of [[star]]s, known as [[Population III]] stars (possibly 70%), and [[dwarf galaxies]] (very early small high-energy galaxies) (possibly 30%).<ref name="popIII_sim">{{Cite journal |last1=Gnedin |first1=Nickolay Y. |last2=Ostriker |first2=Jeremiah P. |date=1997 |title=Reionization of the Universe and the Early Production of Metals |journal=The Astrophysical Journal |language=en |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |issn=0004-637X |s2cid=5758398}}</ref><ref name="qso_z">{{cite arXiv |first=Limin |last=Lu |date=1998 |title=The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium |eprint=astro-ph/9802189 |display-authors=etal}}</ref><ref name="Bouwens_LLG">{{Cite journal |last=Bouwens |first=R. J. |display-authors=etal |date=2012 |title=Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at ''z'' ⩾ 5–8 from the HUDF09 WFC3/IR Observations |journal=The Astrophysical Journal |volume=752 |issue=1 |pages=L5 |arxiv=1105.2038 |bibcode=2012ApJ...752L...5B |doi=10.1088/2041-8205/752/1/L5 |issn=2041-8205 |s2cid=118856513}}</ref><ref name="qso_source1">{{Cite journal |last=Madau |first=Piero |display-authors=etal |date=April 1999 |title=Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources |journal=The Astrophysical Journal |language=en |volume=514 |issue=2 |pages=648–659 |arxiv=astro-ph/9809058 |bibcode=1999ApJ...514..648M |doi=10.1086/306975 |issn=0004-637X |s2cid=17932350}}</ref><ref name="qso_source0">{{Cite journal |last1=Shapiro |first1=Paul R. |author-link=Paul R. Shapiro |last2=Giroux |first2=Mark L. |date=October 1987 |title=Cosmological H II regions and the photoionization of the intergalactic medium |journal=The Astrophysical Journal |language=en |volume=321 |pages=L107 |bibcode=1987ApJ...321L.107S |doi=10.1086/185015 |issn=0004-637X |doi-access=free}}</ref><ref name="qso_source2">{{Cite journal |last=Fan |first=Xiaohui |display-authors=etal |date=December 2001 |title=A Survey of ''z'' > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at ''z'' ~ 6 |journal=The Astronomical Journal |volume=122 |issue=6 |pages=2833–2849 |arxiv=astro-ph/0108063 |bibcode=2001AJ....122.2833F |doi=10.1086/324111 |s2cid=119339804}}</ref>

Quasars show evidence of elements heavier than [[helium]], indicating that galaxies underwent a massive phase of [[star formation]], creating [[population III stars]] between the time of the [[Big Bang]] and the first observed quasars. Light from these stars may have been observed in 2005 using [[NASA]]'s [[Spitzer Space Telescope]],<ref>{{cite web |url=http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |title=NASA Goddard Space Flight Center: News of light that may be from population III stars |publisher=Nasa.gov |access-date=2011-07-01 |archive-date=2011-04-16 |archive-url=https://web.archive.org/web/20110416191522/http://www.nasa.gov/centers/goddard/news/topstory/2005/universe_objects.html |url-status=dead }}</ref> although this observation remains to be confirmed.