A team of astronomers, led by Dr Alex Geringer-Sameth of Carnegie Mellon University, has detected gamma rays emanating from the recently discovered dwarf galaxy Reticulum 2. According to the scientists, such a detection may be the signal of dark matter lurking at the galaxy’s center.
This image shows the newly-discovered dwarf galaxy Reticulum 2. Image credit: Sergey E. Koposov et al / Dark Energy Survey.
The discovery of Reticulum 2 was announced yesterday by a group of astronomers at the Cambridge University’s Institute of Astronomy, UK.
This dwarf galaxy is located in the constellation Reticulum, approximately 97,000 light-years away. It is a very elongated object, around 200 light-years long.
Due to the massive tidal forces of our Milky Way Galaxy, Reticulum 2 is in the process of being torn apart.
“In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature,” said team member Dr Savvas Koushiappas of Brown University.
“It seems like we may now be detecting such a thing for the first time,” he said.
Astronomers have been looking at dwarfs for signs of gamma rays for the last several years using NASA’s Fermi Gamma-ray Space Telescope. There’s never been a convincing signal until now.
Using data from the telescope, Dr Koushiappas, Dr Geringer-Sameth and their colleagues have shown gamma rays coming from the direction of Reticulum 2 in excess of what would be expected from normal background.
“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,” said Dr Geringer-Sameth, who is the first author of the paper accepted for publication in the journal Physical Review Letters (arXiv.org preprint).
“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 astronomers caution that while these preliminary results are exciting, there is more work to be done to confirm a dark-matter origin.
A leading theory suggests that dark matter particles are so-called Weakly Interacting Massive Particles (WIMPs).
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 centers 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.
That’s why dwarf galaxies are important in the hunt for the dark matter particle.
Dwarf galaxies 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.
Further study of Reticulum 2’s attributes could reveal hidden sources that may be emitting gamma rays, but Dr Geringer-Sameth’s team is optimistic.
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Alex Geringer-Sameth et al. 2015. Evidence for Gamma-ray Emission from the Newly Discovered Dwarf Galaxy Reticulum 2. Physical Review Letters, accepted for publication; arXiv: 1503.02320
