Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

Using NASA's Hubble Space Telescope, a team of astronomers has discovered a ring of dark matter that resulted due to a tremendous collision of two giant galaxy clusters.

This is strong evidence that this mysterious form of matter exists. Astronomers have long postulated the existence of the dark matter, to account for the immense gravity that tightly holds the clusters of galaxies.

The nature of dark matter remains unclear, but it is generally believed to be a type of massive, weekly interacting elementary particles that pervade the Universe. Dark matter does emit or reflect light. It can be only observed indirectly, by studying its gravitational influence on the motions of nearby stars or galaxies.

"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, one of the researchers who found the dark-matter ring.

The researchers made the unprecedented discovery while 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 2.6 million light-years across.

Astronomers can also infer the existence of dark matter by observing its effects on the light emitted from more distant objects, a phenomenon known as gravitational lensing. In his theory of General Relativity, Albert Einstein predicted that light is bent when it passes nearby a massive cosmic object. A gravitational lens is observed when a massive galaxy or cluster of galaxies lies in or very near to our line of sight to more distant, background cosmic object, e.g., a galaxy of a quasar. As light is bent in the gravitational field of the foreground object, the image of the background object will tend to be distorted or smeared.

"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," said Jee. "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."

As the team was analyzing the data, they discovered this strange structure that resembles ripples of water. At the first glance, the researchers even thought it was a deformation in the image.

"I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction," Jee explained. "I could not believe my result, but the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I have looked at a number of clusters and I have not seen anything like this."

The discovery of this ring enhances the conclusions of an earlier study that demonstrated that this cluster of galaxies collided with another cluster 1 to 2 billion years ago.

This mass of dark matter appears ring-shaped due to our Earth-based perspective, from which we are viewing the cluster along the line of sight.

The researchers developed computer software to simulate the interactions of galaxy clusters.

"By studying this collision, we are seeing how dark matter responds to gravity," said team member Holland Ford of Johns Hopkins University. "Nature is doing an experiment for us that we cannot do in a lab, and it agrees with our theoretical models."

It is theorized that dark matter makes up most of the mass in the Universe. Ordinary matter comprises only a few percent of the Universe.

First, evidence of the existence of dark matter was discovered in the 1930s by Dr. Fritz Zwicky, when he estimated the mass of a large cluster of galaxies. Zwicky's computations showed that visible matter (galaxies) is only a small fraction of the total mass of the cluster, and an invisible mass dominates the constituents of the cluster. However, his finding was not widely accepted for a long time afterwards.

In some intriguing studies, astronomers study the gravitational affects of dark matter on the shapes of background galaxies. The gravity of dark matter distorts and smears the images of more distant galaxies into arcs and streaks. Gravitational lenses are a powerful natural tool to map the distribution of dark matter in clusters of galaxies. Such maps enable astronomers to deduce the cluster's mass.

"The collision between the two galaxy clusters created a ripple of dark matter that left distinct footprints in the shapes of the background galaxies," Jee explained. "It is like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles' shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring."

Jee and his collaborators applied HST's Advanced Camera for Surveys (ACS) to detect the feeble, distorted, distant galaxies beyond the cluster that cannot be resolved with ground-based telescopes. "Hubble's exquisite images and unparalleled sensitivity to faint galaxies make it the only tool for this measurement," said team member Richard White of the Space Telescope Science Institute (STScI) in Baltimore. The team's paper is due for publication within a few weeks.

Further Reading

Hubble Maps the Cosmic Web of "Clumpy" Dark Matter in 3-D

http://hubblesite.org/newscenter/archive/releases/2007/01/image/a/

Aymen Mohamed Ibrahem

Senior Astronomy Specialist