Newly discovered dwarf galaxy may signal dark matter

A newly discovered dwarf galaxy orbiting the Milky Way appears to be radiating gamma rays, which may signal dark matter lurking at the galaxy's centre, according to a new study.
The galaxy, named Reticulum 2, was discovered earlier this year in the data of the Dark Energy Survey, an experiment that maps the southern sky to understand the accelerated expansion of the universe.
At approximately 98,000 light-years from Earth, Reticulum 2 is one of the nearest dwarf galaxies yet detected.
"Something in the direction of this dwarf galaxy is emitting gamma rays," said Alex Geringer-Sameth, a postdoctoral research associate in Carnegie Mellon University (CMU)'s Department of Physics and the paper's lead author.

"There's no conventional reason this galaxy should be giving off gamma rays, so it's potentially a signal for dark matter," said Geringer-Sameth.
Using publicly available data from NASA's Fermi Gamma-ray Space Telescope, researchers have shown gamma rays coming from the direction of the galaxy in excess of what would be expected from normal background.

"In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature," said Savvas Koushiappas, assistant professor of physics at Brown University.
"It seems like we may now be detecting such a thing for the first time," said Koushiappas.

No one knows exactly what dark matter is, but it is thought to account for around 80 per cent of the matter in the universe.
Scientists know that dark matter exists because it exerts gravitational effects on visible matter, which explains the observed rotation of galaxies and galaxy clusters as well as fluctuations in the cosmic microwave background.
"The gravitational detection of dark matter tells you very little about the particle behaviour of the dark matter," said Matthew Walker, assistant professor of physics and a member of CMU's McWilliams Centre 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," said Walker.

A leading theory suggests that dark matter particles are WIMPs - Weakly Interacting Massive Particles. When pairs of WIMPs meet, they annihilate one another, giving off high-energy gamma rays.
If that's true, then there should be a lot of gamma rays emanating from places where WIMPs are thought to be plentiful, like the dense centres of galaxies.
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, researchers said.
That's why dwarf galaxies are important in the hunt for the dark matter particle. Dwarfs are thought to lack other gamma-ray-producing sources, so a gamma ray flux from a dwarf galaxy would make a very strong case for dark matter.

The research was published in the journal Physical Review Letters.