Editing Einstein ring (section)

=== Known Einstein rings ===
[[File:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg|thumb|"Smiley" or "Cheshire Cat" image of [[galaxy cluster]] (SDSS J1038+4849) and [[gravitational lensing]] (an "Einstein ring") discovered by an international team of scientists,<ref>{{cite journal |last1=Belokurov |first1=V. |display-authors=etal |title=Two new large-separation gravitational lenses from SDSS|journal=Monthly Notices of the Royal Astronomical Society |date=January 2009 |volume=392 |issue=1 |pages=104–112 |arxiv=0806.4188 |bibcode=2009MNRAS.392..104B |doi=10.1111/j.1365-2966.2008.14075.x |doi-access=free |s2cid=14154993 }}</ref> imaged with [[Hubble Space Telescope|HST]]<ref">{{cite web |last1=Loff |first1=Sarah |last2=Dunbar |first2=Brian |title=Hubble Sees A Smiling Lens |url=http://www.nasa.gov/content/hubble-sees-a-smiling-lens/ |date=10 February 2015 |work=[[NASA]] |access-date=10 February 2015 }}</ref>]]

Hundreds of gravitational lenses are currently known. About half a dozen of them are partial Einstein rings with diameters up to an [[arcsecond]], although as either the mass distribution of the lenses is not perfectly [[Axial symmetry|axially symmetrical]], or the source, lens, and observer are not perfectly aligned, we have yet to see a perfect Einstein ring. Most rings have been discovered in the radio range. The degree of completeness needed for an image seen through a gravitational lens to qualify as an Einstein ring is yet to be defined.

The first Einstein ring was discovered by Hewitt et al. (1988), who observed the radio source [[MG1131+0456]] using the [[Very Large Array]]. This observation saw a [[quasar]] lensed by a nearer galaxy into two separate but very similar images of the same object, the images stretched round the lens into an almost complete ring.<ref>{{cite web | url=http://www.nrao.edu/pr/2000/vla20/background/ering/ | title=Discovery of the First "Einstein Ring" Gravitational Lens | publisher=[[NRAO]] | date=2000 | access-date=2012-02-08}}</ref> These dual images are another possible effect of the source, lens, and observer not being perfectly aligned.
[[File:Webb Telescope Detects Universe’s Most Distant Organic Molecules (52958010034).jpg|thumb|[[James Webb Space Telescope|JWST]] false-color image of [[SPT0418-47]], a high-redshift galaxy rich in organic molecules, which appears as a nearly-perfect Einstein ring]]

The first complete Einstein ring to be discovered was [[B1938+666]], which was found by King et al. (1998) via optical follow-up with the Hubble Space Telescope of a gravitational lens imaged with [[MERLIN]].<ref name="Bullseye"/><ref>{{cite news| url=https://query.nytimes.com/gst/fullpage.html?res=9906EFDF103BF932A05750C0A96E958260 | work=The New York Times | title='Einstein Ring' Caused by Space Warping Is Found | first=Malcolm W. | last=Browne | date=1998-03-31 | access-date=2010-05-01}}</ref> The galaxy causing the lens at B1938+666 is an ancient [[elliptical galaxy]], and the image we see through the lens is a dark [[dwarf galaxy|dwarf]] [[satellite galaxy]], which we would otherwise not be able to see with current technology.<ref>{{cite journal|last1=Vegetti|first1=Simona|display-authors=etal|title=Gravitational detection of a low-mass dark satellite at cosmological distance|journal=[[Nature (journal)|Nature]]|date=January 2012|volume=481|issue=7381|pages=341–343|arxiv = 1201.3643 |bibcode = 2012Natur.481..341V |doi = 10.1038/nature10669|pmid=22258612|s2cid=205227095}}</ref>

In 2005, the combined power of the [[Sloan Digital Sky Survey]] (SDSS) with the Hubble Space Telescope was used in the Sloan Lens ACS (SLACS) Survey to find 19 new gravitational lenses, 8 of which showed Einstein rings,<ref>{{cite web|last1=Bolton|first1=A|display-authors=etal|title=Hubble, Sloan Quadruple Number of Known Optical Einstein Rings|url=http://hubblesite.org/newscenter/archive/releases/2005/32/|website=Hubblesite|access-date=2014-07-16}}</ref> these are the 8 shown in the adjacent image. As of 2009, this survey has found 85 confirmed gravitational lenses but there is not yet a number for how many show Einstein rings.<ref>{{cite journal|last1=Auger|first1=Matt|display-authors=etal|title=The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies|journal=The Astrophysical Journal|date=November 2009|volume=705|issue=2|pages=1099–1115|arxiv = 0911.2471 |bibcode = 2009ApJ...705.1099A |doi = 10.1088/0004-637X/705/2/1099 |s2cid=118411085}}</ref> This survey is responsible for most of the recent discoveries of Einstein rings in the optical range, following are some examples which were found:

*[[FOR J0332-3557]], discovered by [[Remi Cabanac]] et al. in 2005,<ref>{{cite journal |last1=Cabanac|first1=Remi |display-authors=etal |title=Discovery of a high-redshift Einstein ring|journal=Astronomy and Astrophysics|date=2005-04-27|volume=436|issue=2|pages=L21–L25|arxiv = astro-ph/0504585 |bibcode = 2005A&A...436L..21C |doi = 10.1051/0004-6361:200500115 |s2cid=15732993 }}</ref> notable for its high [[redshift]] which allows us to use it to make observations about the [[early universe]].
*The "[[Cosmic Horseshoe]]" is a partial Einstein ring which was observed through the gravitational lens of LRG 3-757, a distinctively large Luminous Red Galaxy. It was discovered in 2007 by V. Belokurov et al.<ref>{{cite journal|last1=Belokurov|first1=V.|display-authors=etal|title=The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy|journal=[[The Astrophysical Journal]]|date=December 2007|volume=671|issue=1|pages=L9–L12|arxiv = 0706.2326 |bibcode = 2007ApJ...671L...9B |doi = 10.1086/524948 |s2cid=9908281}}</ref>
*[[SDSSJ0946+1006]], the "double Einstein ring" was discovered by Raphael Gavazzi and Tomasso Treu<ref>{{cite journal|last1=Gavazzi|first1=Raphael|display-authors=etal|title=The Sloan Lens ACS Survey. VI: Discovery and Analysis of a Double Einstein Ring|journal=The Astrophysical Journal|date=April 2008|volume=677|issue=2|pages=1046–1059|arxiv = 0801.1555 |bibcode = 2008ApJ...677.1046G |doi = 10.1086/529541 |s2cid=14271515}}</ref> in 2008, notable for the presence of multiple rings observed through the same gravitational lens, the significance of which is explained in the next section on [[Einstein rings#Extra rings|extra rings]].
Another example is the radio/X-Ray Einstein ring around [[PKS 1830-211]], which is unusually strong in radio.<ref>{{cite journal|last1=Mathur|first1=Smita|last2=Nair|first2=Sunita|title=X-Ray Absorption toward the Einstein Ring Source PKS 1830-211|journal=The Astrophysical Journal|date=20 July 1997|volume=484|issue=1|pages=140–144|arxiv = astro-ph/9703015 |bibcode = 1997ApJ...484..140M |doi=10.1086/304327 |s2cid=11435604}}</ref> It was discovered in X-Ray by Varsha Gupta et al. at the Chandra X-Ray observatory<ref>{{cite web|last1=Gupta|first1=Varsha|title=Chandra Detection of AN X-Ray Einstein Ring in PKS 1830-211|url=https://www.researchgate.net/publication/252412929|website=ResearchGate.net|access-date=16 July 2014}}</ref> It is also notable for being the first case of a quasar being lensed by an almost face-on [[spiral galaxy]].<ref>{{cite journal|last1=Courbin|first1=Frederic|title=Cosmic alignment towards the radio Einstein ring PKS 1830-211 ?|journal=The Astrophysical Journal|date=August 2002|volume=575|issue=1|pages=95–102|arxiv = astro-ph/0202026 |bibcode = 2002ApJ...575...95C |doi = 10.1086/341261 |s2cid=13960111}}</ref>

Galaxy MG1654+1346 features a radio ring. The image in the ring is that of a quasar [[Radio galaxy|radio lobe]], discovered in 1989 by G.Langston et al.<ref>{{cite journal |last1=Langston |first1=G. I. |display-authors=etal |date=May 1989 |title=MG 1654+1346 – an Einstein Ring image of a quasar radio lobe |journal=Astronomical Journal |volume=97 |pages=1283–1290 |bibcode=1989AJ.....97.1283L |doi=10.1086/115071}}</ref>

In June 2023, a team of astronomers led by Justin Spilker announced their discovery of an Einstein ring of distant galaxy rich in organic molecules ([[Aromatic compound|aromatic hydrocarbons]]).<ref>{{Cite journal |last1=Spilker |first1=Justin S. |last2=Phadke |first2=Kedar A. |date=June 2, 2023 |title=Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy |url=https://www.nature.com/articles/s41586-023-05998-6 |journal=Nature |volume=618 |issue=7966 |pages=708–711 |doi=10.1038/s41586-023-05998-6|pmid=37277615 |arxiv=2306.03152 |bibcode=2023Natur.618..708S |s2cid=259088774 }}</ref><ref>{{Cite news |last=Hutchins |first=Shana K. |date=June 5, 2023 |title=Webb Telescope Detects Universe's Most Distant Organic Molecules |work=Texas A&M Today |url=https://today.tamu.edu/2023/06/05/webb-telescope-detects-universes-most-distant-organic-molecules/ |access-date=June 29, 2023}}</ref>

In September 2023, a scientist named Bruno Altieri saw a hint of an Einstein ring in the data coming back from the Euclid space telescope.<ref>{{Cite web |title=Euclid discovers a stunning Einstein ring |url=https://www.esa.int/Science_Exploration/Space_Science/Euclid/Euclid_discovers_a_stunning_Einstein_ring |access-date=2025-02-10 |website=www.esa.int |language=en}}</ref> The ring is located in a galaxy, NGC 6505, that is not too far from Earth–about 600-million light years away.<ref>{{Cite web |author1=Robert Lea |date=2025-02-10 |title=Euclid 'dark universe' telescope discovers stunning Einstein ring in warped space-time (image) |url=https://www.space.com/euclid-einstein-ring-dark-matter |access-date=2025-02-10 |website=Space.com |language=en}}</ref>

In February 2025, the [[Euclid space telescope]] captured a nearly perfect Einstein ring around galaxy [[NGC 6505]], about 590 million light-years away. This gravitational lensing effect bent light from a background galaxy 4.42 billion light-years away.<ref>{{cite web |last=Strickland |first=Ashley |url=https://edition.cnn.com/2025/02/11/science/einstein-ring-euclid-telescope/index.html |title=Space telescope reveals rare 'Einstein ring' phenomenon with startling clarity |publisher=CNN |date=11 February 2025 }}</ref>