"Cosmic Train Wreck" May Derail Theories of Dark Matter

John Roach
for National Geographic News
August 22, 2007
New observations of a massive intergalactic collision are raising more questions about the mysterious nature of dark matter.

The study looked at three huge galaxy clusters that are merging into an even bigger cluster called Abell 520, located about three billion light-years from Earth.

Astronomers describe the collision as a "cosmic train wreck," given the immensity of the forces involved. Each cluster contains about a thousand galaxies, and each galaxy has billions of stars.

The new findings show an unprecedented mix-up in the merging clusters, suggesting the need for an "uncomfortable" revision—or entire rewrite—of our current theories of dark matter.

"Whatever happened did something really unusual to the galaxies," said study lead author Andisheh Mahdavi, an astronomer at the University of Victoria in British Columbia, Canada.

"It moved them all the way on the outer edge of the central region of the cluster, so that only gas and dark matter is left at the center," he added.

"That's never been observed before, and it's really hard to explain."

The findings are slated to appear in an upcoming issue of the Astrophysical Journal.

Mysterious Matter

Dark matter does not absorb or emit light, but scientists believe it makes up about 90 percent of the matter in the universe.

Prevailing theories hold that dark matter is composed of particles that have very weak interactions and move only under the influence of gravity, just like stars.

So when galaxies collide, scientists expect stars and dark matter to move together. Intergalactic gas, however, also responds to pressure and thus is expected to lag behind the other matter.

Observations of the so-called bullet cluster collision reported last year support the conventional theory. (Read the story: "Dark Matter Proof Found, Scientists Say" [August 22, 2006].)

The bullet cluster observations show dark matter and stars moving together and ahead of the intergalactic gas.

But when Mahdavi and colleagues studied Abell 520, they found a dark matter core separated from most of the galaxies.

Eye in the Sky

Mahdavi and colleagues mapped the distribution of matter in Abell 520 with two ground-based optical telescopes in Hawaii and the orbiting Chandra X-Ray Observatory (see more pictures from the Chandra observatory).

Optical telescopes determine the location galaxies from their starlight and infer the location of dark matter by the way its gravity bends the light of other galaxies in the distant background, a technique called gravitational lensing.

X-ray telescopes detect the radiation given off by scorching hot intergalactic gas.

Mahdavi and colleagues plan to re-observe Abell 520 with the Hubble Space Telescope later this year, which will allow for more precise mapping of the dark matter.

If the Hubble observations confirm the distribution of matter in the cluster, astronomers may be forced to revise the physics used to explain the behavior of dark matter, Mahdavi said.

"Uncomfortable" Explanations

There are two possible explanations, "and they're both equally uncomfortable," Mahdavi said.

The galaxies may have been flung to the outer edge of the cluster via a gravitational slingshot, in the same way astronomers can send satellites around a nearby planet for an extra push to the outer solar system.

"But that has a number of problems, in that we haven't been able to make [with computer simulations] slingshots that are powerful enough," Mahdavi said.

The other possibility is the dark matter just got left behind as the galaxies passed on through the center of the cluster, suggesting that the dark matter interacted through some force other than gravity.

"The problem with that is this is not the theory of dark matter that's generally accepted," he said.

The bullet cluster observations, for instance, appear to confirm the accepted theories that dark matter contains only weakly interacting particles.

Wait and See

Avi Loeb is an astronomer at Harvard University in Cambridge, Massachusetts, who was not involved in the study.

He said he will wait for the results of the Hubble observations "before making a judgment whether the nature of dark matter might be different."

If the result is confirmed, researchers will probably try to explain the phenomenon via the gravitational slingshot effect, which is more plausible than completely revising the physics, Loeb said.

"I don't think we should revise our ideas just based on this example, because there is also the bullet cluster that is showing a different picture and clearly the circumstances are different in the two clusters," he said.

Additional studies of other galactic collisions will help firm up conclusions about dark matter, Loeb added.

"At the moment, we have just two examples of these natural laboratories for dark matter dynamics," he noted.

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