MAR 16, 2015 6:29 AM PDT

In Search of Dark Matter

A new galaxy, discovered only weeks ago, has given scientists much to talk about. Physicists at Carnegie Mellon, Brown and Cambridge Universities have analyzed high-energy light coming from the general direction of the new galaxy and have theorized that it might be evidence of dark matter hidden in the center of the galaxy.

The galaxy, which has been dubbed Reticulum 2, was discovered amidst the data of the Dark Energy Survey, a project that maps the sky in an effort to understand the expansion of the universe. The exact source of this high-energy light is uncertain at this point, but researchers feel it's a significant sign of dark matter.
"Something in the direction of this dwarf galaxy is emitting gamma rays. There's no conventional reason this galaxy should be giving off gamma rays, so it's potentially a signal for dark matter," said Alex Geringer-Sameth, a postdoctoral research associate in CMU's Department of Physics and the paper's lead author.
Could a newly discovered galaxy hold clues to dark matter?
Reticulum 2 is one of the nearest dwarf galaxies yet detected. Carnegie Mellon's Geringer-Sameth and Matthew Walker, and Brown's Savvas Koushiappas have shown gamma rays coming from the direction of the galaxy in excess of what would be expected from normal background by analyzing data from NASA's Fermi Gamma-ray Space Telescope,
"In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature," said Koushiappas, an assistant professor of physics at Brown. "It seems like we may now be detecting such a thing for the first time."

Dark matter has long been a scientific mystery. No one knows exactly what it is, but estimates are that it accounts for around 80 percent of the matter in the universe. Scientists know that dark matter exists because the gravitational waves it exerts are visible in many ways. It explains the rotation of galaxies and galaxy clusters as well as the fluctuations in the cosmic microwave background.

"The gravitational detection of dark matter tells you very little about the particle behavior of the dark matter," said Walker, assistant professor of physics and a member of CMU's McWilliams Center for Cosmology. "But now we may have a non-gravitational detection that shows dark matter behaving like a particle, which is a holy grail of sorts."
One theory about dark matter suggests that it consists of WIMPs or Weakly Interacting Massive Particles. When pairs of WIMPs meet, they annihilate one another, giving off high-energy gamma rays. The trouble is, the high-energy rays also originate from many other sources, including black holes and pulsars, which makes it difficult to untangle a dark matter signal from the background noise.

That's why dwarf galaxies are important to scientist looking for dark matter. Dwarfs do not have other sources of gamma rays, so when they are detected, the odds are good that it could be dark matter. "They're basically very clean and quiet systems," Koushiappas said.
Scientists have been looking at them for signs of gamma rays for the last several years using NASA's Fermi Gamma-ray Space Telescope, but until now there has never been a significant signal.
Geringer-Sameth focused his attention on the area of Reticulum 2 and looked at gamma rays coming from there as well as gamma rays coming from adjacent areas of the sky to provide a background level.

"There did seem to be an excess of gamma rays, above what you would expect from normal background processes, coming from the direction of this galaxy," Geringer-Sameth said. "Given the way that we think we understand how gamma rays are generated in this region of the sky, it doesn't seem that those processes can explain this signal."
The team has submitted their analysis to the journal Physical Review Letters, and posted it on arXiv [arXiv:1503.02320].
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