A big find in the hunt for elusive dark matter
Astronomers cite new evidence that the unseen 'glue' holding galaxies together really exists.
Building cosmic maps of matter no one can see may seem like a strange way to make a living. But for astronomer Myungkook James Jee, the work may have netted evidence that seals the case for the existence of dark matter – unseen, "missing mass" first invoked in 1933 to explain why spinning galaxies and galaxy clusters don't tear themselves apart as they twirl.
Dr. Jee and his colleagues announced Tuesday that they have discovered a wispy ring of dark matter around a huge cluster of galaxies 5 billion light-years from Earth. The researchers didn't spot the dark matter directly; it doesn't emit light. Instead, they analyzed the way its gravity bends light from galaxies behind it – an effect called gravitational lensing. If confirmed, the result would rule out other, more exotic notions that have emerged to explain how galaxies and galaxy clusters are held together.
Typically, dark matter's gravity is thought to act as a scaffold for building galaxies and structures that astronomers can detect. That was their only initial explanation for how these structures held up – otherwise the mass in luminous matter wouldn't yield enough gravity to hold galaxies or galaxy clusters together as they rotate. So, astronomers surmised, bright and dark matter generally should go hand in glove.
Some scientists, however, are uncomfortable relying on the unseeable to act as this cosmic glue. For nearly 25 years, they have sought other ways to explain the structures they see. For example, some suggest that you don't need to invoke dark matter if gravity behaves differently at huge spatial scales than it does at small scales. Such alternative explanations couldn't be ruled out – unless someone could find dark matter out there on its own, with a structure unlike that of the nearest clusters.
Jee's discovery gets closer than ever to doing just that. It suggests that the ring of dark matter, 2.6 million light-years wide, is clearly separated from the hot gas and other luminous matter in the cluster. If the result holds up, this stand-alone dark matter would close the door to these other theories, some astronomers say.
It would also sustain a long-cherished assumption underlying space science: Gravity works the same way everywhere in the universe as it does on Earth.
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.