The collision of two clusters of galaxies 5 billion light-years away could help astronomers better understand “dark matter,” the invisible stuff that makes up about a quarter of our universe. Graduate student William Dawson of the University of California, Davis, is leading a team studying the collision, and he discussed it today, Jan. 10, in a talk [number 223.07] at the annual meeting of the American Astronomical Society in Austin, Texas.

“A galaxy cluster is like a little universe, because it has the same matter composition as the whole universe,” Dawson said. “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. It is nicknamed Perry’s cluster, after team member Perry Gee, a UC Davis research scientist who discovered it, or the “Musket Ball” because it is similar to another merging cluster called 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 said.

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 Perry’s cluster by using the Hubble Space Telescope, the Subaru 8-meter telescope in Hawaii, the Kitt Peak National Observatory 4-meter Mayall telescope and the Keck 10-meter telescopes in Hawaii to map the visible galaxies, and the 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 galaxies within the cluster are so vast that an actual collision is 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 said.

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 said — a million-million times bigger than a supernova. Only a handful have been studied to date.

The “Musket Ball” is the furthest progressed of these merging clusters yet seen, Dawson said. 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 said.

# # #

UC Davis colleagues who joined Dawson in his work: Associate Professor David Wittman; project scientist M. James Jee; Professor J. Anthony Tyson; postdoctoral researchers Samuel Schmidt, Paul Thorman and Brian Lemaux; and Assistant Professor Marusa Bradac.

Other team members: John Hughes of Rutgers University; Satoshi Miyazaki and Yousuke Utsumi of The Graduate University for Advanced Studies Tokyo; and Vera Margoniner, California State University, Sacramento.

The National Aeronautics and Space Administration supported the work through Chandra Award No. GO1-12171X issued by the CXO Center, which is operated by the SAO for and on behalf of NASA under contract NAS8-03060NASA, and NASA through a grant from STScI, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555, for Program No. GO-12377

Media Contact:
Andy Fell
+1 (530) 752-4533; cell: +1 (530) 304-8888
ahfell@ucdavis.edu

Science Contacts:
Will Dawson / Dave Wittman
+1 (530) 752-0882; cell: +1 (214) 603-9277 / +1 (530) 754-5354
wadawson@ucdavis.edu / dmwittman@ucdavis.edu