Eight Einstein rings

If you have a lot of mass you can bend light towards you. This was one of the consequences of the theory of general relativity and was first verified in an observation by Sir Arthur Eddington in 1919. He measured the change in position of a star that was seen to the side of the Sun during a solar eclipse. The star appeared to be about 1.75 arcseconds (about a thousandth the angular size of the Sun) closer to the Sun than it would be if the path of light wasn't bent. Basically the Sun was acting like a lens, focussing light. The effect is also predicted by Newton (obviously not personally) but he thought it would only be half that amount. So in the play-off between Einstein and Newton, Einstein came out on top.

All matter will bend light towards it as it passes by, just as the Sun does. If you have a massive galaxy of stars sat between the Earth and an even more distant galaxy, such as a quasar, the light from the quasar is bent around the nearer galaxy. This will usually produce two or four images of the distant quasar around the galaxy but you can also produce lots of interesting arcs of light if you have a cluster of galaxies. If you happen to have the Earth, galaxy and quasar perfectly lined up, you get a beautiful ring of light surrounding the lensing galaxy. This special case was also predicted by Einstein in a paper he published in 1936 and so they get called Einstein rings in his honour.

Adam Bolton, Leon Koopmans and others used the spectra of the light detected in the Sloan Digital Sky Survey to find a list of elliptical galaxies that they think may be candidates for gravitational lenses. They then used the advanced camera for surveys (ACS) onboard the Hubble Space Telescope to have a better look. Amongst the 50 galaxies that they looked at, eight were found to be Einstein rings (the blue arcs in the image). They still have another 50 or so candiate galaxies to look at so they may find some more.

Einstein Rings
Eight Einstein rings found using the HST and the Sloan Digital Sky Survey CREDIT: NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team
Apart from being pretty, gravitational lenses also tell us loads of things about the Universe. You can use them to measure the size of the Universe and also find out about the density of galaxies. This is sort of obvious because you get fewer lenses if there aren't as many galaxies and if you have more galaxies there is more chance that some of them will line-up enough to give you a lens. Measuring the details of the arcs and images lets you also work out how much stuff - both normal matter and dark matter - is in the lensing galaxy so you can effectively weigh it.

You can see the same effect as a gravitational lens if you happen to have a broken wine glass (make sure you have a responsible adult with you ;-). Draw a large dot on a sheet of paper and hold it behind the broken off base of the wine glass. When you line up the neck of the glass with the dot you should get an Einstein ring.

Posted in astro blog by Stuart on Thursday 01st Dec 2005 (21:27 UTC) | Permalink
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