August 22, 2006

Dark matter spied in galactic collision

Galactic crash reveals mysterious particles.

by Geoff Brumfiel
Nature News

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Astronomers have found the strongest evidence so far that mysterious particles that don't interact with light are lurking throughout the Universe.

Doug Clowe from the University of Arizona in Tuscon and his collegaues have spied a cluster of galaxies in which the centre of mass is shifted to the side of the bulk of observable material, in a relatively empty patch of sky. This implies that there must be some 'dark matter' filling up the empty space.

Decades ago, researchers found from watching distant galaxies that their gravitational properties cannot be explained by the visible material: they posited that some 'dark matter' must make up the difference. Dark matter is thought to account for roughly a third of the mass in the Universe, although it has never been directly detected.

But not everyone agrees. Some researchers have questioned whether dark matter exists at all; perhaps, they suggest, the discrepancy seen in the mass of distant galaxies is due to a modification in gravity rather than a mysterious set of particles.

The new observation argues against that.

Mass merger

At visible wavelengths, 1E 0657-56 looks like an average jumble of galaxies. But in the X-ray spectrum, scientists were able to see two massive clouds of dust in between the galaxy clusters, left over from the explosive collision of the original two clusters. These clouds of dust are ten times more massive than the galaxies themselves.

Yet, by observing the way that light from background galaxies is bent by the strong gravitational field of the dust and galaxies in the foreground, the team has concluded that the centre of mass for 1E 0657-56 is shifted out towards the smaller galaxies rather than lying within the more massive dust clouds. So there must be something, they say, filling up the dark patches of these galaxies to lend them extra weight.

"The great thing about this is that you can say that dark matter does exist," says Sean Carroll, a theoretical astrophysicist at the University of Chicago, Illinois.

But not everyone is convinced. A paper accepted by the Monthly Notices of the Royal Astronomical Society1 suggests that modified versions of gravity could still account for the discrepancy.

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