The hunt for mysterious dark matter has so far produced very little results, but scientists might have found evidence of "dark matter annihilation," essentially dark matter particles colliding and producing a signal. An American team of astronomers discovered the promising gamma-ray emission coming from within the halo of the Milky Way.
The source, 3FGL J2212.5+070, was found to be producing gamma-rays by NASA's Fermi telescope. What's unusual, though, is that this source is spread over a large region – or in other words, it is spatially extended. Other sources for gamma-rays, such as pulsars and black holes, are "point-like" and come from one place.
Dark matter annihilations produce light in a limited wavelength range and since this source was not detected at other wavelengths aside from gamma-rays, it is a great candidate for being a small clump of dark matter (known as a subhalo) that is annihilating into gamma-rays. The findings have been submitted to the Journal of Cosmology and Astroparticle Physics, and are available on arXiv.
“Extended objects that emit in gamma-rays also emit in radio, X-rays, and so on,” lead author Dr. Bridget Bertoni told IFLScience. “So, the fact that we found something that was emitting only in gamma-rays that was extended and had a spectrum that looked like a dark matter annihilation spectrum made this source an especially interesting dark matter subhalo candidate.”
NASA's Fermi telescope, illustrated, was used to make the discovery. NASA/Fermi
Dark matter is a mysterious type of matter that permeates the universe. It’s six times more abundant than ordinary matter and it doesn’t interact with light, hence the nickname “dark.” But we can see the effect of dark matter on the shape of galaxies and how they are distributed in space.
Dark matter is thought to accumulate at the center of galaxies, so several searches have been conducted to look for signals from the core of the Milky Way. But while some potential signals have been detected, there’s a lot of debate on the nature of these sources.
“We followed another way to search for this dark matter annihilation, complementary to searches in the galactic center,” said Dr. Bertoni. “We are looking at dark matter subhalos. These are small bound clumps of dark matter that are in the Milky Way. It is predicted that there should be many small clumps of dark matter around galaxies such as the Milky Way.”
Astronomers think that dark matter could be made by weakly interacting massive particles (WIMPs). When these particles collide, they are expected to emit gamma-rays – the signal of which could be observed by gamma-ray telescopes such as the Fermi telescope.
While the source is very promising, the researchers have discussed the possibility that the detection is not a single extended subhalo but two nearby gamma-ray sources. The model suggests that there is a 2 percent probability that a pair such as this would exist somewhere in the sky. The chance is small but not zero, and more observations are necessary to confirm its true nature.
“It would be great to have other follow-up [observations] of this object,” said Dr. Bertoni. “What you would need are observations with a high angular resolution to distinguish if this is two objects or one object. Also, to continue to search for gamma pulsations coming from this object would be useful as well.”