When galaxy clusters collide

UC DAVIS (US) — Researchers have new details about what happens when two clusters of galaxies collide.

“A galaxy cluster is like a little universe, because it has the same matter composition as the whole universe,” says William Dawson, a PhD candidate in physics at the University of California, Davis. “By studying this little universe, we can learn more about our own.”

This “little universe” is formally called DLSCL J0916.2+2951 and consists of two clusters of hundreds of galaxies each, in the process of merging into one.

This image of merging galaxy clusters taken with the Hubble Space Telescope shows that the “North” and “South” clusters have passed through each other. Blue coloring shows dark matter, and red shows gas clouds which have collided in the middle. (Credit: William Dawson, UC Davis)

It is nicknamed Perry’s Cluster, after team member Perry Gee, a UC Davis research scientist who discovered it. It is similar to another merging cluster—the Bullet Cluster—but relatively further along in its development.

Perry’s Cluster comprises about 86 percent dark matter, 12 percent superheated gas and 2 percent actual stars. Those proportions are similar to the distribution of mass in the universe as a whole, Dawson says.

Dark matter is thought to interact very little, if at all, with “regular” matter and does not emit light. But it does exert a gravitational pull on light passing through or near it, distorting the image of distant objects—rather like looking through the bottom of a glass bottle.

Dawson and colleagues mapped the visible galaxies in Perry’s Cluster by using the Hubble Space Telescope, the 8-meter Subaru Telescope in Hawaii, the 4-meter Mayall Telescope at the Kitt Peak National Observatory in Arizona, and the twin 10-meter telescopes at the W.M. Keck Observatory in Hawaii, and the orbiting Chandra X-ray Observatory for the super-hot gas.

And, with the Hubble, Subaru and Mayall telescopes, the researchers mapped the dark matter by observing distortions in light passing through the cluster from more distant objects—a method called weak gravitational lensing.

The map revealed that the two galaxy clusters within Perry’s Cluster had passed through each other—the spaces between the galaxies within the clusters are so vast that actual collisions are unlikely—and that most of the dark matter also had passed through without collision.

Not so with the gas clouds; they had collided and become stuck between the clusters, making a huge cloud of gas a thousand times hotter than the surface of the sun.

“Because these mergers separate the various matter components of the cluster, they provide astronomers with dissection of the cosmos that would otherwise be impossible,” Dawson says.

By comparing and contrasting the behavior of the dark matter to that of the galaxies and gas in the merging cluster, physicists can rule out some theories about dark matter’s properties.

The energy of these merging clusters is incomprehensibly large, Dawson explains—a million-million times bigger than a supernova. Only a handful have been studied to date.

The cluster is the most advanced of these merging clusters yet seen, Dawson notes. Every great advance in our understanding of the physical world is the direct result of understanding how things change with time, so the hope is that by observing clusters at different stages of merging, astronomers can gain insight into the physics involved, he says.

Collaborators include researchers at Rutgers, the Graduate University for Advanced Studies Tokyo; and California State University, Sacramento. The team presented the findings Jan. 10 in a press briefing at the annual meeting of the American Astronomical Society in Austin, Texas.

The National Aeronautics and Space Administration supported the work through the Chandra X-Ray Observatory Center and through a grant from Space Telescope Science Institute.

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