An Einstein ring

One hundred years ago, Einstein developed his theories of relativity (Special first and General later) which helped describe extreme events that Sir Isaac Newton had no experience of. Apart from telling us that the speed of light was a constant (in a vacuum), they also told us about space-time and how it interacts with matter.

When you stick a lump of mass in empty space it distorts the space around it in the same way that a lead ball bends a rubber sheet, although this is in three dimensions rather than just two. The distortion of space also affects the travel of light; a light beam can be deflected as it goes past a large mass. You can think of the mass (say a galaxy or cluster of galaxies) as a sort of gigantic lens that focuses the light from distant objects. Indeed, these objects are known as gravitational lenses for that very reason. The first gravitational lens, 0957+561, was found in 1979 by Dennis Walsh, Bob Carswell, and Ray Weymann. This radio object was seen to coincide with a close pair of faint blue fuzzies which both turned out to be the same distant quasar.

Gravitational lenses can actually tell you an awful lot of information about the distant quasar as well as the galaxy that acts as the lens. One of the most exciting things you can do is actually work out the distance to the quasar by watching how the images, created by the lensing galaxy, vary. Calculating distances may seem straightforward on the Earth, but they are traditionally very difficult to measure accurately out in the Universe as you can't just use a tape measure. To help work out some of these properties, many people carry out searches to find new gravitational lenses. Although the most obvious gravitational lenses have been discovered - they are the easiest to find - many searches are still ongoing.

Now occasionally the quasar, galaxy and Earth will just happen to line up. When this occurs a circular ring of images, of the distant quasar, are formed around the lensing galaxy. This beautiful distortion of light is known as an Einstein ring in honour of the gentleman whose theories helped explain them. The most famous example is an object named 1938+666 which has been imaged in both the radio and optical parts of the spectrum. Now, astronomers using the VLT in Chile have seen another. This one is known as FOR J0332-3557 and is in the southern constellation of Fornax.

Einstein Ring
IMAGE: Einstein ring seen with VLT/FORS1, ESO.

The image above is a composite made from two colour bands (B and R). You can see the ring in the centre of the image surrounded by a whole host of interesting objects (click the image above to see a full-size version), some as faint as magnitude 26. The lensing galaxy (the dot in the centre) is about 8,000 million light-years away from us and the distant quasar (forming the ring) is about one and a half times that. That is a LONG way indeed.

Posted in astro blog by Stuart on Monday 04th Jul 2005 (18:26 UTC) | Permalink
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