30TH MARCH, 1998
A BULLS--EYE FOR MERLIN AND THE HUBBLE
A team of British astronomers using the UK's MERLIN radio
array and the Hubble Space Telescope have found an "Einstein
Ring" - a gravitational effect predicted by Albert Einstein over 60
years ago as a consequence of his General Theory of Relativity.
The Hubble picture is a beautiful demonstration of Einstein's
ideas since, for the first time, it shows a complete ring
surrounding the galaxy that created it.
The effect is a cosmic mirage caused by the gravity of a massive galaxy bending the light from an object behind it and acting as a "gravitational lens". On the rare occasions when the distant object, the lens galaxy and the telescope are exactly aligned an "Einstein ring" is created.
Dr. Ian Browne of the University of Manchester admits, "At first sight it looks artificial and we thought it was some sort of defect in the image but then we realised we were actually looking at a perfect Einstein ring!"
Commenting on the pictures, Bristol University astronomer Professor Mark Birkinshaw said, "MERLIN and the Hubble have scored a bulls--eye!".
The size of the ring on the sky is tiny - roughly a second of arc or about the size of a penny viewed from a distance of over two miles - even though the lens consists of an entire galaxy. The blurring effect of the atmosphere makes such fine detail hard for astronomers to spot using optical telescopes on the Earth.
The British team found it by using the 135-mile-wide MERLIN radio telescope to image distant radio sources. MERLIN is a network of six radio telescopes spread out across England and operated as a national facility by the University of Manchester's Jodrell Bank observatory. MERLIN's resolution is the same as that of the Hubble Space Telescope but at a completely different wavelength - the two make perfect astronomical partners. The Hubble, orbiting above the atmosphere, took a detailed picture of the object and this revealed the spectacular bulls-eye. This is only one of over 20 galaxy lenses now known.
In an ironic twist, counting the number of gravitational lenses in the sky, including the rare Einstein rings, is the best way of seeing whether Einstein really made his "greatest blunder". When he applied his General Theory of Relativity to the Universe as it was known 80 years ago, Einstein had to invent a repulsive force which overcomes gravity at very large distances. This new force was soon dismissed by other astronomers but many modern cosmologists now think that Einstein may have been right first time - the lens searches will soon tell us where the truth lies.
The radio picture produced by MERLIN (see press release WWW page, Figure 1), which allowed the system to be recognised in the first place, shows only part of a ring. The reason is that, while the source of radio emission is embedded in the distant galaxy, it is not exactly aligned with the lens galaxy. The "optical" picture produced by the Hubble (see press release WWW page, Figure 1) is actually in the infrared region of the spectrum, taken with the NICMOS camera. The wavelength used is about twice that of red light. The infrared emission from the distant galaxy is more extended than the radio emission. Some of it comes from directly behind the lens galaxy and hence a complete ring is formed.
The way in which a gravitional lens produces multiple images, including the special Einstein ring case, is illustrated in the explanatory diagram (Figure 2 on the press release WWW page).
Einstein's "greatest blunder" refers to the elusive Cosmological Constant. This describes the strength of the long-range repulsive force he introduced into the General Relativity equations in 1916. Other astronomers soon showed, however, that this force was not needed to explain the properties of the Universe as it was then known. Einstein ruefully wrote "away with the cosmological term". But like a genie, once released it has proved hard to put away and many astronomers now invoke the Cosmological Constant to account for modern observations of the distant universe.
A Universe in which the Cosmological Constant is not identically zero has different geometrical properties to one governed solely by gravity. Counting gravitational lenses, in other words counting the number of lines-of-sight "blocked" by intervening galaxies, is acknowledged to be the best way of measuring the geometry of the Universe at large distances. By the end of this year we expect to be able to place the best limit so far on the Cosmological Constant.
The astronomers involved work at the following institutes : University of Manchester; University of Oxford; California Institute of Technology (Pasadena); Netherlands Foundation for Radio Astronomy; University of Groningen; Institut d'Astrophysique de Paris.