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Galactic Collision Challenges Dark Matter Theories

17 August 2007 (All day)
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NASA/CXC/M. Weiss

Split up.
The Abell 520 cluster collision ripped apart galaxies from dark matter (blue) and hot gas clouds (magenta). The circles show where astronomers had expected the galaxies to end up.

Astronomer Arthur Eddington's prophetic quote about the universe still holds: It is stranger than we can imagine. And new images from NASA's Chandra X-ray Observatory combined with views from two ground-based optical telescopes have produced one of the biggest cosmic mysteries yet. A collision between giant galactic clusters seems to have separated the galaxies from their dark matter cores. The find threatens to turn current thinking about dark matter on its ear.

Dark matter was first predicted 75 years ago, when astronomers noticed that the Milky Way's stars moved faster than they should given the galaxy's mass. Ever since, scientists have searched for evidence of the mysterious substance, which is thought to compose about 25% of the universe. (Visible matter accounts for only 5%, and the even more mysterious dark energy makes up the rest.) So far, they have only been able to detect dark matter's gravitational influence. Clumps of the stuff seem to pull a galaxy together. So, it comes as a major surprise that the new Chandra and ground images apparently show a giant zone of dark matter and a bunch of galaxies going their separate ways.

The images depict hundreds of galaxies merging into a huge cluster called Abell 520, located about 2.4 billion light-years away. As astronomer Andisheh Mahdavi of the University of Victoria in Canada and colleagues will report in the 20 October issue of The Astrophysical Journal, some of the galaxies have moved as far away as 2 million light-years from their dark matter anchors, far enough that gravity will never bring them back. Perhaps just as incredible, the clouds of hot interstellar gas formerly contained by the galaxies--and superheated by the collision so they glow in x-ray light--seem to have been grabbed by the dark matter instead of being flung into space.

The scene contrasts with that of a similar collision reported a year ago in the bullet cluster (ScienceNOW, 21 August 2006). There, dark matter could be seen clearly holding on to the galaxies, not the gas. Abell 520 suggests that something not yet understood has acted to split the galaxies away from dark matter after the collision. The problem, Mahdavi says, is that computer simulations haven't yet been able to duplicate the effects seen in Abell 520, so for now, the phenomenon remains a complete mystery.

Joshua Barnes of the University of Hawaii, Manoa, agrees: "This sort of separation is not supposed to happen." One possible explanation, Barnes says, is that the configuration is more complex than it appears in the images. Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says it's better to wait for new observations before revising ideas of dark matter. Mahdavi's team plans to look at Abell 520 with the Hubble Space Telescope soon in hopes of gleaning further clues.

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