Hubble Finds Dark Matter Ring in Galaxy Cluster
Astronomers using NASA's Hubble Space Telescope have discovered a ghostly ring of dark matter that formed long ago during a titanic collision between two massive galaxy clusters. The ring's discovery is among the strongest evidence yet that dark matter exists. Astronomers have long suspected the existence of the invisible substance as the source of additional gravity that holds together galaxy clusters. Such clusters would fly apart if they relied only on the gravity from their visible stars. Although astronomers don't know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the universe. "This is the first time we have detected dark matter as having a unique structure that is different from both the gas and galaxies in the cluster," said astronomer M. James Jee of Johns Hopkins University in Baltimore, Md., a member of the team that spotted the dark-matter ring. The researchers spotted the ring unexpectedly while they were mapping the distribution of dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located 5 billion light-years from Earth. The ring measures million light-years across. Although astronomers cannot see dark matter, they can infer its existence in galaxy clusters by observing how its gravity bends the light of more distant background galaxies. "Although the invisible matter has been found before in other galaxy clusters, it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters," Jee said. "By seeing a dark-matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter." During the team's dark-matter analysis, they noticed a ripple in the mysterious substance, somewhat like the ripples created in a pond from a stone plopping into the water. "I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our
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