A strange, never-before-seen glow in the halo of our galaxy may be the strongest dark-matter breadcrumb yet.
A new analysis of 15 years’ worth of data from the Fermi Gamma-Ray Space Telescope reveals a glow of unusually high-energy gamma rays that cannot easily be attributed to any known source.
According to astronomer Tomonori Totani of the University of Tokyo in Japan, it may be the radiation produced when hypothetical dark matter particles collide and wipe out one another.
It’s not the first time astronomers have gone looking for such a glow – but it’s the first time one has been found peaking at this specific energy level in the galactic halo, the large bubble of gas and radiation that surrounds the Milky Way.
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“We detected gamma rays with a photon energy of 20 gigaelectronvolts (or 20 billion electronvolts, an extremely large amount of energy) extending in a halolike structure toward the center of the Milky Way galaxy,” Totani explains.
“The gamma-ray emission component closely matches the shape expected from the dark matter halo.”
Dark matter is one of the enduring mysteries of the Universe. It manifests as ‘excess’ gravity that can’t be attributed to the sum of matter we can see.
Scientists calculate that normal matter accounts for only about 16 percent of the matter distribution of the Universe, with the remaining 84 percent consisting of dark matter whose identity is unknown.
One of the leading candidates for dark matter is a hypothetical class of particles called weakly interacting massive particles, or WIMPs. Current theory suggests that, when WIMPs and their antiparticles collide, they annihilate each other, a process that produces a shower of different particles, including gamma-ray photons we may actually see.
This brings us back to our newest breadcrumb. If we can detect a gamma-ray glow with no clearly identifiable source, it’s possible that that glow was generated by dark matter annihilation.
Scientists have conducted searches to this effect, but the results so far are inconclusive.
One particular region of interest is the galactic center, where the dark matter density is believed to be particularly high, and the signal of its presence ought to be accordingly strong. Indeed, hints of a gamma-ray excess have been found there.
The galactic halo, by contrast, is a relatively under-explored region in the search for a dark matter annihilation signal. Any such signal would be much fainter than a signal from the galactic center, making it much more difficult to detect in the first instance.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>However, the halo isn’t packed with gamma-ray sources like the millisecond pulsars thought to be scattered throughout the galactic center, making any potential signal cleaner.
To overcome the faintness problem, Totani needed several solutions. The first was an extraordinary dataset: 15 years of observations collected by the Fermi Large Area Telescope.
Because the halo is so dim, gamma-rays are relatively few and far between. A significant number of their photons would be required to perform the statistical analysis capable of revealing an excess signal. In addition, a larger dataset could increase the signal-to-noise ratio, making the data more reliable.
Totani compared this data to known sources of gamma-ray emission in the galactic halo, such as the Fermi bubbles and point sources. Whatever gamma-ray emission remained after accounting for all these known sources was compiled into a map.
That resulting map showed a large, spherical, halo-like region of faint gamma-ray emission with a peak at 20 gigaelectronvolts – within the predicted range for WIMP annihilation. That’s far from a smoking gun, but it’s tantalizing enough to warrant further investigation.
“If this is correct, to the extent of my knowledge, it would mark the first time humanity has ‘seen’ dark matter,” Totani says. “And it turns out that dark matter is a new particle not included in the current standard model of particle physics. This signifies a major development in astronomy and physics.”
Well, maybe. A lot more work needs to be done to verify the finding, including independent analyses of the data to try to replicate it, investigations to determine whether other astrophysical processes may produce the same glow, and searches of other environments, such as dwarf galaxies, for similar halos.
All this is going to take time, probably years.
Still, a gamma-ray excess with the energies and shape predicted for dark matter annihilation is an interesting step forward towards an answer to the dark matter question first posed by Swiss Astronomer Fritz Zwicky nearly a century ago.
The research has been published in the Journal of Cosmology and Astroparticle Physics.
