The Musket Ball Cluster, so named because it is older and slower moving than the Bullet Cluster, represents the aftermath of two galaxy clusters which moved through each other, pulled by gravity. This particular system is important largely because it is one of few known collisions and, of those known, it is the farthest along its collision path.
Galaxy clusters are made up of three components: hundreds of galaxies, gas a thousand times hotter than the surface of the sun and dark matter. Due to the large distance between galaxies, they do not actually interact as the clusters pass through each other. However, the hot gas is the one component that does collide, remaining in the center of the collision as momentum carries the clusters away from each other.
“The main purpose of studying merging clusters is to understand about dark matter,” said Perry Gee, a research specialist at UC Davis, who discovered the system.
Researchers are particularly interested in how the dark matter interacts in the system — or more to the point, how it does not. The dark matter of both galaxy clusters has not interacted with each other and remains surrounding their respective galaxies post-collision. By studying the Musket Ball Cluster system, William Dawson, a fifth-year Ph.D. student and head of the project, hopes to contribute to the developing understanding of dark matter.
Studying dark matter may seem abstract to some, but as one studies science, the future applications are not always clear.
“The hope is that it will be like Einstein’s [theory of] General Relativity, which one hundred years ago, when he came up with it, there was no application for,” theorizes Dawson, “But now, we’re completely dependent upon his theoretical work — our GPS systems would not work without these corrections.”
Scientists don’t yet know how knowing about dark matter could affect the future, but as Dawson said, “Maybe our grandchildren’s grandchildren will figure out some use for this dark matter, which makes up roughly 25 percent of the entire universe.”
Besides constraining the ideas of dark matter, there are several other important applications of studying the Musket Ball Cluster. Observing this system could help us to understand the evolution of galaxies based on changes to their environment, cluster systems as accelerators of high energy cosmic rays and whether the universe is composed of only matter (or perhaps matter and antimatter).
“If one [cluster] was composed of matter and the other antimatter, we would see lots of gamma rays as the matter and antimatter particles annihilate,” David Wittman, the project supervisor said. “The fact that we do not … lends support to the idea that the entire universe is made of matter.”
Actual discovery of this system began in 1999 when Wittman used the Deep Lens Survey to scan the sky along with Tony Tyson of UC Davis and Ian Dell’Antonio of Brown University. It was not until 2006 that UC Davis researcher Gee discovered the system, which was first unrecognized by the less advanced cluster finding equipment.
In 2007, Gee passed the project on to Dawson who had more time to devote to the research.
A total of six telescopes were used to discover and map the system: the Hubble Space Telescope, the Subaru 8m Telescope, the KPNO 4m Mayall Telescope, the Keck 10m Telescope, the Chandra Space Telescope and the Sunyaev-Zel’Dolvich Array.
ALEX STANTON can be reached at email@example.com.