A dwarf galaxy orbiting the Milky Way is humming with gamma rays, leading astronomers to hypothesize that it could be filled with annihilating dark matter particles.
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Dark matter is thought to make up for 85 percent of all the mass in the known universe. By looking at the spin of galaxies and the interactions inside galactic clusters, an invisible type of matter exerts a powerful gravitational pull on local space. But because dark matter does not interact with normal (‘bayonic’) matter via the electromagnitic force, it cannot be seen and therefore its nature remains highly mysterious.
One leading hypothesis of the source of dark matter is that of weakly-interacting massive particles, or WIMPS. These particles have yet to be discovered, but if they are out there they could account for the bulk of the universal missing mass.
WIMPS, when they collide, are theorized to annihilate. One of the byproducts of this annihilation is energy in the form of gamma rays. But simply scanning the cosmos for sources of WIMP-generated gamma rays is a difficult task as there are many other phenomena out there that also generate gamma ray radiation.
That’s where dwarf galaxies come in.
Known gamma ray sources include black holes and pulsars, making the identification of annihilating WIMPs a precarious task. But ancient dwarf galaxies are known to lack a large number of these sources, making them key hunting grounds for the signature of dark matter. “They’re basically very clean and quiet systems,” said Savvas Koushiappas of Brown University in a press release.
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“In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature,” he added. “It seems like we may now be detecting such a thing for the first time.”
Located only 98,000 light-years away, the dwarf galaxy Reticulum 2 is one of the closest dwarf galaxies found to date and it has become the focus of the dark matter search. Through the analysis of observational data from NASA’s Fermi Gamma-ray Space Telescope, Koushiappas and Carnegie Mellon University colleagues Alex Geringer-Sameth and Matthew Walker realized that Reticulum 2 was generating gamma rays at a rate far higher than any known gamma ray source.
“Something in the direction of this dwarf galaxy is emitting gamma rays,” said Geringer-Sameth, the study’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.”
Seeking the nature of dark matter has been maddeningly difficult, so when a predicted signature of the annihilation of WIMPs in a dwarf galaxy right next door to the Milky Way is revealed, it’s understandable that some excitement, and guarded caution, is shown.
“The gravitational detection of dark matter tells you very little about the particle behavior of the dark matter,” said Walker. “But now we may have a non-gravitational detection that shows dark matter behaving like a particle, which is a holy grail of sorts.”
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In a paper submitted to the journal Physical Review Letters (available on the arXiv preprint service), Geringer-Sameth, Koushiappas and Walker make it clear that it is currently unknown why there’s an excess of cosmic rays pumping out of Reticulum 2, but it could also be signs of the predicted signature generated by a concentration of WIMPs inside the dwarf galaxy.
“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 Geringer-Sameth. “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 fact that there are gamma rays and also a clump of dark matter in the same direction makes it quite interesting,” Walker added.
This is not the first time that dwarf galaxies have been tapped as possible dark matter factories, nor is Reticulum 2 the only place where tentative dark matter signals have been detected, but it is another tantalizing clue that we are getting closer at understanding what dark matter is, or — if another source for gamma rays inside dwarf galaxies is found — isn’t.
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