New Way to Peer at Distant Galaxies

The cosmic lenses, discovered with the European Space Agency's Herschel Space Observatory, allow astronomers to see galaxies which are otherwise too distant to study. This provides key insights into how galaxies have changed over the history of the cosmos. Herschel looks at far-infrared light, which is emitted not by stars, but by the gas and dust from which they form.

The results, which are from the very first data taken as part of the Herschel-ATLAS project, the largest imaging survey conducted so far with Herschel, are published in the scientific journal Science on November 5).

Dr Mattia Negrello, of the Open University and lead researcher of the study, explained, "Our survey of the sky looks for sources of sub-millimetre light. The big breakthrough is that we have discovered that many of the brightest sources are being magnified by lenses, which means that we no longer have to rely on the rather inefficient methods of finding lenses which are used at visible and radio wavelengths."

The Herschel-ATLAS images contain thousands of galaxies, most so far away that the light has taken billions of years to reach us. Dr Negrello and his team investigated five surprisingly bright objects in this small patch of sky. Looking at the positions of these bright objects with optical telescopes on the Earth, they found galaxies that would not normally be bright at the far-infrared wavelengths observed by Herschel. This led them to suspect that the galaxies seen in visible light might be gravitational lenses magnifying much more distant galaxies seen by Herschel.

At the University of Hertfordshire, Dr David Bonfield used visible and near-infrared measurements taken from existing visible images to estimate distances to the galaxies believed to be acting as lenses. He explained, "More distant galaxies have their light stretched to longer, redder wavelengths, because the light travels through more of the expanding Universe before reaching us. So we can use the colours they appear to figure out how far away they are."

To find the true distances to the Herschel sources behind the lenses, Negrello and his team looked for a tell-tale signature of molecular gas. Using radio and sub-millimetre telescopes on the ground, they showed that this signature implies the galaxies are being seen as they were when the Universe was just 2-4 billion years old -- less than a third of its current age. The galaxies seen by the optical telescopes are much closer, each ideally positioned to create a gravitational lens.

Dr Negrello said: "Previous searches for magnified galaxies have targeted clusters of galaxies where the huge mass of the cluster makes the gravitational lensing effect unavoidable. Our results show that gravitational lensing is at work in not just a few, but in all of the distant and bright galaxies seen by Herschel."