Top: The galaxy cluster CL 0024+17 (ZwCl 0024+1652) as seen by Hubble Space Telescope's Advanced Camera for Surveys.
Bottom: A ghostly ring of dark matter in the same galaxy cluster, as inferred by observations of the cluster made by Hubble Space Telescope.

It's out there. Dark matter. You've probably read about it in the papers, or heard scientists talk about it. Nobody's quite sure exactly what it is, yet. But, we do know it's out there, distributed in clumps throughout the galaxies in the universe.

Tracking down dark matter, which is one of the great "Holy Grails" of astronomy and astrophysics these days, is no easy matter (so to speak). Dark matter doesn't give off light, it doesn't reflect light. It just sits there, making its presence known by the gravitational effect its mass has on light. To find it, astronomers study how the light from more distant galaxies (beyond or "behind" the dark matter clumps) is distorted and smeared into arcs and streaks by the gravity of the dark matter. The smears are produced by a phenomenon called "gravitational lensing."

Today a team of astronomers announced that they found a RING of dark matter at the heart of a galaxy cluster. An ordinary "visible light" image of the cluster shows galaxies and some smears of light that are distorted images of galaxies behind the cluster. The smears indicate that there's something in the cluster causing the light from the distant galaxies to bend or distort. That "something" turns out to be a ring of dark matter. Now, if much of the dark matter we've "seen" so far in the universe is clumpy, how does a ring of it end up at the heart of a galaxy cluster?

Good question.

According to one of the astronomers, Dr. M. James Jee of Johns Hopkins University in Baltimore, MD, the collision between two galaxy clusters created a much larger cluster, but it also formed a ripple of dark matter separate from the gas and dust components of the cluster that left distinct footprints in the shapes of the background galaxies. "It's like looking at the pebbles on the bottom of a pond with ripples on the surface," he said. "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."

This is the first time such a distinct ring of dark matter has been seen, and at first Jee did not want to believe the ring-like structure was anything but a statistical fluke in the data analysis. But, like any good scientist does, he systematically went through the data, and eventually was convinced it was real.

"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 couldn't 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've looked at a number of clusters and I haven't seen anything like this."

It's really quite a cosmic detective story. Dark matter is sort of like the "last frontier" of matter studies in some ways. We know it's ubiquitous in the universe, and we're now starting to trace out its distribution, its "shapes" (if you will). There's more dark matter than "regular" (so-called "baryonic" matter in the universe) and it serves as a sort of gravitational glue to bind galaxies and clusters together.

If you want to read more about this story, and see some very cool animations, go to the Space Telescope Institute website and visit their news center. And stay tuned. I think there's going to be more news about this "binding force" of dark matter as scientists do more cosmic mapping!

Labels: cosmology, dark matter, galaxy clusters, gravitational lensing

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