Einsteinring

An Einstein ring ( also Chwolson ring after Orest Danilowitsch Chwolson ) is a ring of electromagnetic radiation from a distant object that is created by the effect of gravity in a “foreground” galaxy . The galaxy acts as a gravitational lens . This phenomenon was predicted by Albert Einstein in his general theory of relativity .

With a gravitational lens, the observer sees the distant object multiple times because the light rays can reach him in different ways. If the object is precisely behind the lens, the images appear as ring segments around the galaxy - under ideal conditions, a complete ring can even be created.

theory

Formation of a complete Einstein ring (a) and two distorted images (b) of a distant object through a foreground galaxy acting as a gravitational lens. The image that appears to an observer on earth is drawn in orange.

Be there

${\displaystyle G}$the gravitational constant ,

${\displaystyle M}$the mass of the object acting as a lens,

${\displaystyle c}$the speed of light ,

${\displaystyle d_{LS}}$ the distance between the lens and the imaged object,

${\displaystyle d_{L}}$ the distance from the observer to the object acting as a lens,

${\displaystyle d_{S}}$ the total distance from the observer to the depicted object,

${\displaystyle r}$ the Einstein radius in meters.

The radius of an Einstein ring is called the Einstein radius . According to Einstein's theory of relativity, it can be calculated as follows. ${\displaystyle r}$

${\displaystyle {\frac {r}{d_{L}}}+{\frac {r}{d_{LS}}}={\frac {4GM}{c^{2}}}{\frac {1}{r}}}$

Reshaped according to results ${\displaystyle r}$

${\displaystyle r={\sqrt {{\frac {4GM}{c^{2}}}\;{\frac {d_{LS}\cdot d_{L}}{d_{S}}}}}}$.

The Einstein radius of an Einstein ring in radians is ${\displaystyle \theta _{E}}$

${\displaystyle \theta _{E}={\sqrt {{\frac {4GM}{c^{2}}}\;{\frac {d_{LS}}{d_{L}\cdot d_{S}}}}}}$.

If the lens and the depicted object are not on one line, so that instead of an Einstein ring, several individual images are created, then the distances between these images are of the order of magnitude of the Einstein radius. In this more frequent case, the Einstein radius gives an idea of ​​the extent of the effect produced by the gravitational lens.

For actually observed examples of the gravitational lensing effect, the approximate size of the Einstein radius is:

Gravitational lens	depicted object	Einstein radius
Galaxy clusters	distant galaxy	a few tens of arc seconds
Galaxy	distant galaxy	a few tenths of an arc second to a few arc seconds
Star in the Milky Way	distant star in the Milky Way	about 0.001 arc seconds, see microlens effect

Observations

From the Earth is only a few Einstein rings are seen as the source of radiation behind the gravitational lens must be accurate. In the Einstein Rings observed so far, the foreground galaxies are a few billion light years away . The apparent diameter of the Einstein rings is between 0.3 " and 2".

The first Einstein ring ( MG 1131 + 0456 ) with a diameter of 1.75 ″ was recorded with the VLA in 1987 (publication 1988). More than 70 Einstein rings had been found by 2008. Many have been spotted during sky surveys, some of which were examined further with the Hubble Space Telescope :

• 

  LRG 3-757

• 

  SDSS J162746.44 005357.5 , diameter 2.08 ± 0.08 "

• 

  SDSS J120540.43 491029.3

A double Einstein ring was discovered in 2008. Such a double Einstein ring occurs when two galaxies are at different distances behind the lens galaxy at the same time. The probability of such a constellation is around one in ten thousand. In this case, there were three galaxies three, six and eleven billion light years apart. The geometry of the recorded objects allows conclusions to be drawn about the force of gravity and thus the mass of the distracting (middle) galaxy, which could be determined to be one billion solar masses :

• 

  Double Einstein ring SDSS J0946 1006

Further examples

• B1938 + 666 : Was published in 1992 by Patnaik et al. discovered as a gravitational lens system. An Einstein ring was later discovered in this lens with the “ Merlin ” (Multi-Element Radio Linked Interferometer), a network of radio telescopes in Great Britain and the Hubble space telescope (King et al., 1997/1998).
• SDSS J143001.30 + 410440.6
• In 2018, an Einstein ring was measured on the nearby galaxy ESO 325-G004. By determining the mass of this galaxy, Einstein's theory of relativity could be confirmed with an accuracy of 9%. The European Southern Observatory (ESO) and the Hubble telescope were used .
• The galaxy MACS J7017.5 + 3745 forms one of the largest known Einstein rings .

See also

• Einstein Cross

Web links

Commons : Einsteinringe  - collection of images, videos and audio files

• Merlin WebSite of the Jodrell Bank Radio Observatory
• Press release with pictures of the discovery of several Einstein rings in 2005 (English)
• Report on the discovery of a double Einstein ring. In: NZZ , January 13, 2008

Individual evidence

1. ^ R. Gavazzi et al .: The Sloan Lens ACS Survey. VI: Discovery and analysis of a double Einstein ring . In: Astroph. Journ . tape 677 , 2008, p. 1046-1059 , bibcode : 2008ApJ ... 677.1046G .
2. ↑ Märkische Oderzeitung , January 15, 2008, p. 18
3. ↑ Theory of relativity in the nearby galaxy tested orf.at, June 22, 2018, accessed June 23, 2018.
4. ↑ VLT makes the most precise test of Einstein's general theory of relativity outside the Milky Way eso1819de - Science press release, eso.org, June 21, 2018, accessed June 23, 2018.
5. ↑ Adi Citrine, Tom Broadhurst, Yoel Rephaeli, Sharon Sadeh: "The Largest Gravitational Lens: MACS J0717.5 + 3745 (z = 0.546)", The Astrophysical Journal , 707: L102-L106, 2009 December 10. (pdf)