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Astronomers have the first direct proof that dark matter exists

University of Arizona astronomers and their colleagues got side-on views of two merging galaxy clusters in observations made with state-of-the-art optical and X-ray telescopes.

"Nature gave us this fantastic opportunity to see hypothesized dark matter separated from ordinary matter in this merging system," said UA Steward Observatory astronomer Douglas Clowe, leader of the study.

"Prior to this observation, all of our cosmological models were based on an assumption that we couldn't prove: that gravity behaves the same way on the cosmic scale as on Earth," Clowe said. "The clusters we've looked at in these images are a billion times larger than the largest scales at which we can measure gravity at present, which are on the scale of our solar system."

Clowe added, "What's amazing about this is that the process of galaxy clusters merging is thought to go on all of time. That's how galaxy clusters gain mass. But the fact that we caught this thing only 100 million years after it occurred -- so recently that it barely registers on the cosmic time scale -- is tremendous luck."

Astronomers have known since the 1930s that most of the universe must be made up of something other than normal matter, the stuff that makes stars, planets, all things and creatures. Given the way that galaxies move through space and scientists' understanding of gravity, astronomers theorize that the universe must contain about five times more dark matter than normal matter.

But for the past 70 years, no one had any direct empirical evidence that dark matter even exists.

"Astronomers have been in the somewhat embarrassing position of saying that we understand the universe, although more than 80 percent of it is something we don't know anything about," said UA astronomy Professor Dennis Zaritsky, a member of the discovery team.

"Either most of the matter in the universe is in some invisible, undiscovered form we call 'dark matter' that causes galaxies to move as they do, or we just don't understand the fundamental laws of gravity," Zaritsky said.

When galaxy clusters merge, the galaxies themselves are so sparsely scattered in space that they don't collide, Clowe said. "Even if two galaxies do pass through each other, the distance between the stars is so great that even stars won't collide. Galaxies basically plow through each other almost without slowing down."

Most of a galaxy cluster's normal mass is in its diffuse hot gas. Galaxy clusters typically contain 10 times as much ordinary mass in gas as in stars. So when galaxy clusters merge, the hot gas from each cluster exerts a drag force on the other, slowing all the gas down, Clowe said.The upshot is that the galaxies themselves continue speeding through space, leaving the gas behind.

Observations made with NASA's Chandra X-ray Observatory showed the bulk of ordinary matter is in the hot gas clouds left in the wake of the galaxies. Part of this million-degree plasma of hydrogen and helium, the part from the smaller cluster, forms a spectacular bullet-shaped cloud because a bow shock, or supersonic shock wave, is created in the 10 million mph collision.

But when the astronomers mapped the region of the sky around the galaxies in optical light, they discovered far more mass near the galaxies, ahead of the gas cloud. They analyzed gravitational lensing of distant galaxies in images taken with NASA's Hubble Space Telescope, the European Southern Observatory's 2-meter Wide-Field Imager and one of the twin 6.5-meter Magellan telescopes that a consortium that includes UA operates in Chile.

Gravitational lensing is a phenomenon caused by gravity bending distant starlight. When the astronomers analyzed the shapes and patterns of the distorted light, they discovered the mass of non-luminous, or dark, matter that causes the lensing is far greater than the mass of ordinary matter in the gas cloud.

Clowe and Zaritsky said that dark matter particles are not expected to interact with either normal matter or dark matter particles except through gravity. Hence, they would pass through the collision just as galaxies do.

"We see that dark matter has careened through the collision efficiently," Zaritsky said.

"We're actually using this system to test the idea that dark matter particles are collisionless," Clowe said.

"The bottom line is, there really is dark matter out there," Zaritsky said. "Now we just need to figure out what it is."

The team is publishing the research in a forthcoming issue of the Astrophysical Journal Letters. In addition to Clowe and Zaritsky of UA's Steward Observatory, team members include Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology in Stanford, Calif., Anthony Gonzalez of the University of Florida, and Maxim Markevitch, Scott Randall and Christine Jones of the Harvard-Smithsonian Center for Astrophysics.

Lori Stiles | University of Arizona
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