80
Cosmic mirages
created, based on properties such as the light distribution measured from the images. (The luminous matter is taken as a first indicator of how the underlying dark matter might be distributed.) Then a computer is used to simulate what would happen to different light-rays travelling from different background objects through the cluster ± a process called ray tracing. The model is modified until it reproduces the image of the real thing. An enormous amount of information can be extracted from the Hubble Space Telescope images, enabling constraints to be placed on the model. Further modifications are introduced until the model produces a replica of the image. By this method it is possible to fairly well define the properties of the gravitational lens. The Hubble Space Telescope image of Abell 2218 was a bonus for this type of work, since it shows a rare event: the appearance of multiple images of individual galaxies. An unprecedented seven multiple images were found. With strong lensing, each set of multiple images offered a chance to measure the mass of the galaxy cluster. This was possible because each individual image had a different `impact parameter' ± the distance between the line of sight passing through the lens and the background object, and the image. This distance varies depending on the mass of the intervening object. Since the light rays from each background galaxy take a different path through the foreground cluster, each one gives an independent measure of the mass of the foreground cluster. It is therefore possible to calculate the amount of mass that lies between the apparent location of the image and the true line of sight. What is important about multiple images is that they do not all lie at the same distance from the centre of the foreground cluster. With arclets visible at different places within the image, it is suddenly possible to measure the mass of the foreground cluster at different distances from the centre. The second result from the image was the determination of the distance to the background galaxies. The lensing equations depend on the geometry of the whole affair: the distance between the source, the lens and the observer. The distance to the lensing cluster is easy to determine from redshift observations. What is not known is the distance from the lensing cluster to the background source. However, once the nature of the lens is determined from the computer simulations (described above) it is possible to calculate the distance to the background galaxies. Knowing the distance of the background galaxies from the lens, and thus from ourselves, it is possible to determine how long ago the light from the background galaxies left them ± the look-back time. This allows an understanding of the properties of galaxies at that epoch. Since this now famous image was obtained, astronomers have calculated the approximate distances of 120 arclets, allowing determination of the distances to galaxies fifty times fainter than those visible to ground-based telescopes. A gravitational lens can therefore be used as an enormous telescope to study objects that would otherwise be beyond the reach of even the largest groundbased telescopes. Without gravitational lenses, the distant galaxy clusters would be barely visible; but gravitational lenses such as Abell 2218 boost their visibility by factors of 10±50. The distant galaxies beyond Abell 2218 lie at a redshift of 1±