Herschel Highlights Cosmic Zoom Lenses
NOORDWIJK, The Netherlands, Nov. 10, 2010 — Astronomers using the European Space Agency’s Herschel Space Observatory have discovered a new way of finding cosmic zoom lenses, which allow astronomers to peer at galaxies in the distant universe. The magnification allows astronomers to see galaxies otherwise hidden from us when the universe was only a few billion years old. Herschel looks at far-infrared light, which is emitted not by stars, but by the gas and dust from which they form. This provides key insights into how galaxies have changed over the history of the cosmos.
Every schoolchild is taught that light travels in straight lines, but a century ago Albert Einstein showed that gravity can cause light to bend. The effect is normally extremely small, and it is only when light passes close to a very massive object such as a galaxy containing hundreds of billions of stars that the results become easily noticeable. When light from a very distant object passes a galaxy much closer to us, its path can be bent in such a way that the image of the distant galaxy is magnified and distorted. These alignment events are called “gravitational lenses” and many have been discovered over recent decades, mainly at visible and radio wavelengths.
The effect of gravitational lensing as seen by Herschel and ground-based telescopes. The light from a distant galaxy (red) is warped and magnified by the presence of a foreground galaxy (blue). These effects make the distant galaxy appear much brighter, as seen by Herschel as the orange dot in the upper-most panels. The lensing also distorts the image observed by the Submillimeter Array (SMA), shown here in pink. The foreground galaxy is not seen by Herschel, but is observed by optical telescopes such as the W. M. Keck Observatory on Hawaii, shown in blue. The light from the distant galaxy has been travelling for 11 billion years, allowing astronomers to observe the precursors to galaxies like our own in the very early universe. The effect of the gravitational lensing can be used to work out the structure of both galaxies, probing the formation of stars and galaxies billions of years ago and the role of mysterious dark matter in the relatively nearby universe.
As with a normal glass lens the alignment is crucial, requiring the position of the lens — in this case a galaxy — to be just right. This is very rare and astronomers must rely on chance alignments, often involving sifting through large amounts of data from telescopes. Most methods of searching for gravitational lenses have a very poor success rate, with less than one in ten candidates typically being found to be real. Herschel’s panoramic imaging cameras have allowed astronomers to find examples of these lenses by scanning large areas of the sky in far-infrared and submillimeter light. These results are from the very first data taken as part of the “Herschel-Atlas” project, the largest imaging survey conducted so far with Herschel, and are published in the journal Science.
“Our survey of the sky looks for sources of submillimeter 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,” said Mattia Negrello of the Open University in Milton Keynes, U.K., who is the lead researcher of the study.
The Herschel-Atlas images contain thousands of galaxies, most so far away that the light has taken billions of years to reach us. 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.
To find the true distances to the Herschel sources, Negrello and his team looked for a tell-tale signature of molecular gas. Using radio and submillimeter telescopes on the ground, they showed that this signature implies the galaxies are being seen as they were when the universe was just 2 billion to 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. Negrello said that “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.”
(Photo: NASA/European Space Agency/W. M. Keck Observatory/Submillimeter Array)
The magnification provided by these cosmic zoom lenses allows astronomers to study much fainter galaxies, and in more detail than would otherwise be possible. They are the key to understanding how the building blocks of the universe have changed since they were in their infancy.
“This relatively simple technique promises to unlock the secrets of how galaxies like our Milky Way formed and evolved. Not only does the lensing allow us to find them very efficiently, but it helps us peer within them to figure out how the individual pieces of the jigsaw came together, back in the mists of time,” said Rob Ivison of the Royal Observatory, Edinburgh, part of the team that created the images.
“What we’ve seen so far is just the tip of the iceberg,” said Loretta Dunne of Nottingham University and joint leader of the Herschel-Atlas survey. “Wide area surveys are essential for finding these rare events, and since Herschel has only covered 1/30 of the entire Herschel-Atlas area so far, we expect to discover hundreds of lenses once we have all the data. Once found, we can probe the early universe on the same physical scales as we can in galaxies next door.”
“We can also use this technique to study the lenses themselves,” said Steve Eales of Cardiff University, the other leader of the survey. “This is exciting because 80 percent of the matter in the universe is thought to be dark matter, which does not absorb, reflect or emit light and so can’t be seen directly with our telescopes. With the large number of gravitational lenses that we’ll get from our full survey, we’ll really be able to get to grips with this hidden universe.”
The data from the area of sky used for this work has now been released to the astronomical community. “We hope that now astronomers not directly involved in H-Atlas will dive into this data set and exploit the wealth of science which is bursting to be done with it,” Dunne said.
For more information, visit: www.h-atlas.org
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