12th March 1998

A Bull's Eye for MERLIN and the Hubble

Background Information

The observations

Because of the blurring effect of the atmosphere on optical telescopes, the astronomers use high resolution radio telescopes - the Very Large Array in New Mexico and the MERLIN array in the UK (see the main MERLIN WWW page at - to pick out gravitational lens systems. Only about one in every five hundred distant radio sources (galaxies and quasars) is lensed and so thousands of radio sources have to be searched to have a good chance of success. The British team, working together with an international team of colleagues, have now found thirteen such systems - more than doubling the number previously known.

The radio picture produced by MERLIN (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 r adio emission is embedded in the distant galaxy, it is not exactly aligned with the lens galaxy. The ``optical'' picture produced by the Hubble (Figure 1) is actually in the infra-red region of the spectrum taken with the NICMOS camera. The wavelength used is about twice that of red light. The infra-red 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.

Gravitational Lensing

Unlike the lenses with which we are familiar, in spectacles for example, a gravitational lens can produce not one but several images of a given object; these images may be highly distorted and magnified. Whereas a conventional glass or plastic lens has a simply curved shape the analogy with a gravitational lens is a piece of glass shaped like the base and stem of a wine glass with the bowl cut off. Even without breaking the glass the ring effect can easily be seen by tipping the glass and looking at a mark on a piece of paper (or a table cloth) through the base.

The way in which a gravitional lens produces multiple images, including the special Einstein ring case, is illustrated in the explanatory diagram (Figure 2).

Why study gravitational lenses?

By studying this and other gravitational lenses astronomers can not only measure the masses and shapes of distant galaxies, including any ``dark'' matter which will not show up in the optical or radio pictures, but also can measure Hubble's constant which is related to the time elapsed since the Big Bang.

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.