Dark Matter Ring Detected by Hubble

John Roach
for National Geographic News
May 15, 2007
An international team of astronomers has mapped what appears to be a ring of dark matter around a massive galaxy cluster located some five billion light-years from Earth.

"We believe that this is the strongest evidence yet for the existence of dark matter," James Jee of Johns Hopkins University in Baltimore, Maryland, said today in a telephone briefing with reporters.

Dark matter is a mysterious substance that scientists believe makes up most of the matter in the universe. But because it neither emits nor reflects light, its existence is inferred through indirect means. (See a computer-simulated picture of dark matter.)

The new finding adds to a growing body of evidence that dark matter exists.

Last August another team of scientists said that they detected dark matter in a galaxy cluster called the bullet cluster.

The new evidence comes from a galaxy cluster called ZwC10024+1652. The Hubble Space Telescope took an image of the cluster in 2004.

Astronomers mapped the distribution of dark matter through observations of how gravity bends the light of more distant background galaxies in the image. This technique is called gravitational lensing.

Jee explained that while the dark matter itself is invisible, it distorts the light coming from the background galaxies somewhat, like a ripple of water passing over pebbles at the bottom of a pond.

As the astronomers made the map, they noticed the dark matter formed a ghostly ring around the galaxy cluster. The ring measures 2.6 million light-years across.

The map also shows that dark matter is distributed within the cluster.

Richard White, a team member from the Space Telescope Science Institute in Baltimore, explained at the briefing that the technique allows scientists to determine the distribution of all the matter in the galaxy cluster.

"We can have great confidence that that ring is there and that it really is quite a different distribution than the rest of the matter of the cluster," he said.

Collision Simulated

Previous research showed that the galaxy cluster formed when two distinct clusters collided one to two billion years ago. That is: Had there been humans on Earth with a Hubble-like telescope one to two billion years ago, astronomers would have been able to witness the collision in process.

Jee and colleagues created a computer simulation of the smash-up to determine what happened when the galaxy clusters collided.

In the simulation, as the clusters smashed together, the dark matter fell to the center of the combined cluster and then sloshed back out.

As the dark matter sloshed out, however, the pull of gravity caused it to slow down and pile up like cars bunched up on a highway, the team explained.

"We conclude that the collision that happened about one or two billion years ago is responsible for the creation of the dark matter ring," Jee said.

A paper on the finding will appear in the June 20 issue of the Astrophysical Journal.

Finding Questioned

Robert Massey is a postdoctoral scholar at the California Institute of Technology and an expert on dark matter. He was not part of the research team, but he was included in the telephone briefing to offer perspective.

He said the new finding, if it stands up to scrutiny, adds further proof for the existence of dark matter and will advance scientific understanding of its mysterious nature.

"However, I do have to say this result is meeting substantial skepticism amongst the astronomical community," he noted.

The galaxy cluster itself is not well understood, he said, and some are concerned the dark matter ring is faint.

The gravitational lensing technique used to detect it may also have been compromised by peculiarities of the camera on Hubble, he added.

"So it's really exciting if it's right," Massey said, "but to be convinced of the ring, astronomers really want to see some independent observations verifying it."

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