PHOTO IN THE NEWS: Cluster Smashup Is Dark Matter Proof

colliding galaxies picture
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August 27, 2008—If the latest image from the Hubble Space Telescope looks familiar, that's because this candy-hued galaxy cluster could be a twin of the so-called bullet cluster, a formation hailed in 2006 as the first direct proof for the existence of dark matter.

At 5.6 billion light-years from Earth, the new cluster—known as MACS J0025.4-1222—is much farther away, and thus older, than its famous relative. But the formation shows the same separation of dark and ordinary matter occurring as its two parent galaxy clusters collide at high speed.

As in the bullet cluster, a composite image from optical and x-ray telescopes shows visible matter (pink) in MACS J0025.4-1222 slowing down during the collision.

Meanwhile, most of the cluster's mass (blue) keeps up speed and passes right through the visible matter, creating two clumps that are moving away from the collision. These clumps, astronomers think, are dark matter.

The discovery shows that the bullet cluster is no anomaly, helping to cement the case for dark matter's presence while adding to astrophysicists' toolkit.

"The unusual configuration of this cosmic collision enables astronomers to study mysterious, invisible dark matter," study team member Richard Massey of the University of Edinburgh said of MACS J0025.4-1222 in a press release.

—Victoria Jaggard

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