A big find in the hunt for elusive dark matter

Astronomers cite new evidence that the unseen 'glue' holding galaxies together really exists.

from the May 16, 2007 edition

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The results "are suggestive," acknowledges Douglas Clowe, an Ohio University astronomer who is not part of Jee's team. They don't help identify the exotic subatomic particles dark matter is made of. But if confirmed, the results would "remove the last conspiracy theories against dark matter."

Jee, an astronomer at The Johns Hopkins University in Baltimore, notes that at first he didn't believe his results. His team was mapping the distribution of dark matter in the galaxy cluster, dubbed CL0024+17, using the Hubble Space Telescope's Advanced Camera for Surveys. When the team saw the ring in the data, members suspected it came from flaws in a computer program they'd used. "We spent nearly a year searching for the cause of the problem," Jee says. The more the group tried to remove the ring, the more distinctly it showed up.

Then the group came across an earlier study of the cluster indicating that 1 billion to 2 billion years ago the collection of galaxies collided with another galaxy cluster. The team realized it was viewing the remnants of the collision end-on. The dark-matter ring was, in effect, a ripple expanding out from the collision.

Last year, Dr. Clowe and colleagues reported the first direct evidence for dark matter in another cluster that had endured a collision. But in terms of testing alternative ideas on missing mass, it provided only half a loaf, he explains. The collision had stripped the cluster of its hot gas, so the dark matter was decoupled from that batch of luminous matter – which accounts for most of a cluster's luminous mass. But the dark matter still appeared to be associated with the galaxies in the cluster. Jee's team apparently has found dark matter that has been fully pushed out of the nest.

The findings are controversial, notes Cal Tech astronomer Richard Massey, who this year published a map of dark matter embracing some 500,000 galaxies. The latest measurements come from only one instrument; other studies combine optical images with data taken at other wavelengths. Moreover, "the signal is very weak. Some people are not yet convinced it's more than an artifact," he notes.

Follow-up studies may have to await the launch of the James Webb Space Telescope, slated for 2013. The Hubble camera Jee's team used blew a fuse and ceased operation in January, and it's not clear whether a space shuttle mission can repair it.

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