Distorted Galaxies Point to Dark Matter

16 March 2000 7:00 pm

Never have so many astronomers been so eager to claim they can't see straight. Groups working with three different telescopes have detected weak lensing, a distortion of distant galaxies that reveals the dark matter strewn across space. The results provide the first direct glimpse of the gargantuan tangle of massive, invisible stuff that astronomers and astrophysicists believe makes up the vast majority of the universe.

Galaxies billions of light-years away appear in the sky as faint, luminous ellipses. Gravity from intervening dark matter deflects the light from the galaxies, slightly squashing the ellipses, so that instead of being randomly oriented, the ellipses in any small patch of sky tend to point in the same direction. Astronomers have struggled for years to detect the tiny alignment. Then, in a span of just a few days, three groups rushed to report they had spotted it.

On 27 February, a team working on the Canada-France-Hawaii Telescope (CFHT) in Hawaii posted a preprint detailing its results on Astro-Ph, an Internet server maintained by Los Alamos National Laboratory in New Mexico. Within 5 days, groups working with the William Herschel Telescope in the Canary Islands and with the Cerro Tololo Inter-American Observatory's Blanco telescope in Chile followed suit. On 7 March, the CFHT group released a press release claiming to have spied weak lensing first.

To see the gravitational effect, the astronomers first had to filter out similar but much larger distortions caused by the atmosphere and optical imperfections in the telescope. For that, they turned to stars within our own Milky Way galaxy. At such close range, weak lensing could not affect the stars' images; any distortions must be due to optical and atmospheric effects. So the researchers calculated how to turn the blurred stars back into points and then adjusted the shapes of the galaxies in the same patch of sky the same way. They thus isolated the alignment due to dark matter--an alignment so slight that each group surveyed tens of thousands of galaxies to see it.

The evidence for weak lensing "is beyond the 'Maybe this is real' level," says Timothy McKay, an astrophysicist at the University of Michigan, Ann Arbor, and precision measurements should come quickly. Cosmologist Michael Turner of the University of Chicago says researchers hope to measure the ripples and undulations in the intergalactic tangle of dark matter. "What we'd really like to see is the cosmic infrastructure, the web of dark matter that holds the universe together," he says. "And weak lensing is the way to do it."

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