Such systems are expected to be extremely rare. It will give us insights we can't get from other types of lenses

Dr David Rusin, University of Pennsylvania

It is a unique configuration, produced by the gravitational effect of three galaxies along the line of sight between the more-distant galaxy and Earth.

"This is the first gravitational lens with more than four images of the background object that is produced by a small group of galaxies rather than a large cluster of galaxies," said Dr David Rusin, of the University of Pennsylvania, US.

Gravitational lensing - first predicted by Albert Einstein - is a neat trick that allows scientists to probe distant parts of the Universe in greater detail than would normally be possible (see box).

Astronomers say that the six-way cosmic lens will reveal new information about how galaxies interact with each other.

'Interesting case'

The gravitational lens, called CLASS B1359+154, consists of a galaxy more than 11 billion light-years away in the constellation Bootes. It is positioned behind a trio of galaxies more than seven billion light-years away.

It was found using the Very Long Baseline Array (VLBA) radio telescope in the US and the Hubble Space Telescope (HST) in orbit.

View from a ground-based telescope

The more-distant galaxy shows signs that it contains a massive black hole at its core and also has regions in which new stars are forming.

The gravity of the intervening galaxies has caused the light and radio waves from the single, more distant galaxy to be deflected to form six images as seen from Earth. Four of these images appear outside the triangle formed by the three intermediate galaxies and two appear inside it.

"This lens system is a very interesting case to study because it is more complicated than lenses produced by single galaxies, and yet simpler than lenses produced by clusters of numerous galaxies," said Chris Kochanek, of the Harvard-Smithsonian Center for Astrophysics, US.

"When we understand this system, we will have a much clearer picture of how galaxies are changed by being part of a bigger cluster of galaxies," he added.

Multi-wavelength survey

B1359+154 was discovered in 1999 by the Cosmic Lens All-Sky Survey, an international collaboration of astronomers using radio telescopes to search the sky for gravitational lenses.

Images made by the Very Large Array (VLA) in New Mexico and by Britain's Jodrell Bank radio telescopes showed six objects suspected of being gravitational-lens images, but the results were inconclusive.

Gravitational lensing

When foreground objects are so massive they can bend the light from more distant galaxies passing by them, much as an optical lens bends light to form an image on a photographic plate. The process magnifies, brightens and distorts images of the distant galaxies, and acts like a powerful "zoom lens" for viewing regions of the cosmos that are so far away they could not normally be observed with the largest available telescopes

Rusin and his team used the VLBA and HST in 1999 and 2000 to make more detailed studies of B1359+154. The new data convinced the astronomers that it consists of six lensed images of a single background galaxy.

"This is a great example of modern, multi-wavelength astronomy, said Rusin. "We need the radio telescopes to detect the gravitational lenses in the first place, then we need the visible-light information from Hubble to show us additional detail about the structure of the system."

Armed with the combined VLBA and HST data about the positions and brightnesses of the six images of the background galaxy as well as the positions of the three intermediate galaxies, the astronomers carried out computer simulations to show how the gravitation of the three galaxies could produce the lens effect.

They were able to design a computer model of the system that produced the six images seen in B1359+154.

"The next big step is to use HST to see the pattern of rings produced by the galaxy surrounding the black hole. We already see hints of them, but with the upgrades to HST in the next servicing mission we should be able to trace it completely," Kochanek said.