Strong Evidence of Dark Matter Found

type size: [A] [A] [A]

May 15, 2007 — The strongest evidence yet for the existence of dark matter has been spotted 5 billion light-years away in the ongoing collision of two clusters of galaxies. Astronomers have detected what appears to be a vast ring of dark matter — so wide that even light would need 2.6 million years to cross it.

The dark matter has been detected indirectly by how it bends light — which it does with its gravity. By definition, dark matter emits no light of any kind, but does exert a gravitational pull, like normal matter.

Astronomers were using the Hubble Space Telescope to map the way light from distant objects was bent by the merging cluster (named ZwC10024+1652), when they stumbled onto the ring.

They describe the ring as sort of a circular ripple in the light coming from the cluster. The ripple is rather like the visual distortion seen in a pond when a ripple caused by a stone passes by and contorts the view of pebbles on the bottom.

"Dark matter is the crystal clear water and the pebbles are the structures in the background," explained Myungkook James Jee of Johns Hopkins University in Baltimore, at a Hubble Space Telescope news conference on Tuesday. Jee is a member of the team that discovered the dark matter ring.

As for how the dark matter was pushed into a ring, that’s something the team may have worked out with a computer simulation of the galactic cluster collision.

In that virtual version of the gigantic encounter, dark matter falls to the center of the collision, and then rebounds back out. As it moves outward, it’s slowed down by the gravity of matter all around it. That creates an outward-bound dark matter traffic jam, which can be seen as the ring.

The whole thing looks like a ring to us on Earth because we’re seeing the collision head-on, rather than from the side.

"We see a ring-like structure that is distinct from the galaxies," Jee said.

Another cluster collision in what’s called the Bullet Cluster was reported last year and also showed some separation of dark matter, but it was a side view of the same sort of event. It’s very useful to now have a second view from a 90-degree different perspective, said astronomer Richard Massey of the California Institute of Technology.

"It’s really exciting if it’s right," said Massey.

For one thing, having a clear case showing where dark matter is separated from what we consider ordinary matter may make it possible to study dark matter by itself, which might lead to important clues to what exactly the enigmatic stuff is.

Massey and other researchers would like some independent researchers to check and confirm the discovery. However, that’s not going to be easy to do, since the instrument on Hubble that was used for the discovery — the Advanced Camera for Surveys — stopped operating last year.

It’s possible a future space shuttle mission might repair the Hubble instrument, Massey said. If not, there are some other satellite observatories in the works which might be able to continue the dark matter research.