Detecting Dark Matter No Longer a Massive Task

5 January 2007 (All day)

Canada-France-Hawaii Telescope Corporation 2006

Skewed view.
A small group of galaxies, visible at the center, creates distorted images of distant galaxies far behind it, visible as faint blue streaks.

Even small clusters of galaxies are massive enough to distort spacetime in ways that can be directly observed by astronomers, according to a new study. The finding--and the technology that made it possible--may help researchers better understand the elusive dark matter that makes up most of the stuff of the universe.

Einstein predicted that massive objects, such as the sun, distort spacetime and bend light, just like lenses do. In 1919, astronomer Arthur Eddington confirmed the theory when measurements he took during a solar eclipse indicated that a star passing close to the sun appeared at a slightly different position than it should have. It wasn't until 1979, however, when a new type of image detector called a charge coupled device (CCD) picked up the faint double-image of a quasar, that astronomers were able to directly observe huge astronomical objects acting as a "gravitational lens."

Recently, astronomers have used gravitational lensing to detect dark matter. The more massive an object, the more it distorts spacetime. Thus, researchers can estimate the mass of a galaxy cluster, for example, by measuring how much it skews light from galaxies passing by it. When the mass estimates don't match up with the observed size of the cluster, the difference is assumed to be due to dark matter (ScienceNOW, 15 April 2002).

Until now, however, CCD instruments were only sensitive enough to pick up such lensing effects when very massive objects, such as clusters of hundreds or thousands of galaxies, were involved. A new type of ultrasensitive, high-resolution CCD camera may change this picture. When astronomers at the Hawaii-based Canada-France-Hawaii Telescope (CFHT) atop the summit of Mauna Kea combined this new instrument with automated pattern-recognition software that scanned the CCD images for lensing effects, they detected 43 gravitational-lens candidates in an area of the sky about 100 times that covered by the moon. Among these candidates were a number of smaller galaxy groups with as few as 5 to 50 members. "We were surprised that we could find gravitational lenses around these groups of galaxies because we thought they would not be sufficiently massive to produce such effects," says team member Jean-Paul Kneib of the Astrophysics Laboratory of Marseille, France.

Astronomers should be able to use the new software and CCD technology to detect dark matter in places they weren't previously able to look, says Matthias Bartelmann, an astronomer at Heidelberg University in Germany. "We are now able to find gravitational lenses in substantial numbers--and are able to do real statistical studies," adds astrophysicist Michael Strauss of Princeton University in New Jersey. "As these [lensing effects] become more numerous, the case [for dark matter] becomes stronger."

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