BioEd Online

A ghostly ripple spotted within a cluster of galaxies 5 billion light years away supplies further evidence that the mysterious substance known as dark matter really exists.

An international team of astronomers says that the galactic cluster C1 0024+17 contains a ring of dark matter 2.6 million light years across¹. They were initially so perplexed by the ring that they thought it was just an error in their data.

Dark matter is thought to make up more than 90% of the mass of the Universe. Although it can't be seen directly, astronomers have long thought it must pervade galaxies because of the gravitational influence it exerts on visible matter. No one knows what it consists of, although it must be different stuff from all known subatomic particles.

Usually dark matter follows much the same distribution as visible matter, rather like the way the human population, invisible in satellite images of the night-time Earth, mirrors the visible distribution of artificial light. This means it's hard to tell whether the dark matter exists at all, or whether its apparent effects are caused by a change in the way gravity acts at galactic scales.

Those doubts were salved when, last year, astronomers found that a cluster of galaxies named 1E 0657-56 seems to have a lopsided distribution of visible and total mass (see 'Dark matter spied in galactic collision'. This lopsidedness could best be explained by assuming it is balanced by invisible dark matter, which was separated from the visible hot gas by the collision of two clusters that formed 1E 0657-56 long ago.

In C1 0024+17, again the dark matter seems to have a physical distribution different from the visible stars and gas — whereas the visible matter is in a blob, some of the dark matter is in a distinct ring.

It's this unusual separation of visible and dark matter that provides the clinching evidence for the latter, says Myungkook Jee, one of the observing team and an astronomer at Johns Hopkins University in Baltimore, Maryland. "The more different the distributions, the harder it is to explain them any other way."

Unreal ring

Jee says that when their analysis of images taken by the Hubble Space Telescope seemed to reveal the ring, he couldn't believe the result.

"It took more than a year to convince myself that it was real," he says. "I've looked at a number of clusters and I haven't seen anything like this."

The researchers figure that the ring is a ripple caused by a collision between two clusters 1 to 2 billion years ago, which formed C1 0024+17.

Much the same process caused the separation of dark and some visible matter in 1E 0657-56. But that collision can only be seen side-on from Earth, whereas C1 0024+17 is face-on, making the ring shape visible.

The researchers could infer the presence of the dark-matter ring by looking at how the starlight coming from galaxies behind it is bent. "It's like looking at the pebbles on the bottom of a pond with ripples on the surface," says Jee. "The pebbles' shapes appear to change as the ripples pass over them."

The researchers say the structure of the ring suggests that dark matter doesn't interact strongly with itself except by gravity. This challenges recent suggestions that particles of dark matter might feel each other via another, unknown force. This putative force could be very small at best, Jee and colleagues say, because otherwise interactions between the particles of dark matter would have scattered them. "Any exchange of energy between the dark matter particles by collisions will easily destroy the ring," says Jee.