Gamma rays are found in surprisingly high concentrations in the center of the Milky Way.
The center of our galaxy is behaving strangely, and it may be due to dark matter. In 2009, observations from the Fermi Gamma-ray Space Telescope revealed a surprisingly high concentration of gamma rays emanating from the middle of the Milky Way, a phenomenon called galactic center gamma-ray excess (GCE). Now simulations hint that these gamma rays could result from the annihilation of dark matter particles.
Debate about what might cause GCE has raged since it was first noticed, and two main explanations have emerged over the years. First, it could come from an as-yet unobserved population of pulsars, which are rapidly spinning neutron stars that emit rays powerful radiation.
A second explanation is that this could be due to weakly interacting massive particles (WIMPs), which have long been a leading candidate for inclusion in dark matter. These particles would hardly interact with particles of ordinary matter at all, but if two of them accidentally collided with each other, they would annihilate and create a burst of gamma rays.
The dark matter explanation has fallen out of favor in recent years, in part because direct searches for WIMPs have turned up nothing. “Given the lack of direct evidence for the existence of dark matter, despite very sensitive searches, a higher burden of evidence is required to interpret dark matter,” says Jeff Grube at King's College London.
Another reason this interpretation has fallen by the wayside is that we expect dark matter at the center of a galaxy to take on a spherical shape, and the GCE is not spherical, but rather flattened in the middle. However, new simulations Joseph Silk from Johns Hopkins University in Maryland and his colleagues note that this may not be a problem.
These simulations took into account history of the Milky Way in more detail than previous GCE studies. “We know that billions of years ago our galaxy underwent a merger with a number of smaller galaxies, and they brought their dark matter with them,” Silk says. “The implication of this story is that no one in their right mind would expect the center of the galaxy to be spherically symmetric.”
The results confirmed this, yielding a flattened distribution of dark matter that matched the shape of the GCE. This brings the interpretation of dark matter back into the discussion. But the mystery is not yet solved – pulsars are still a viable explanation. “The situation remains ambiguous at best,” Grube says.
Our current gamma-ray observatories are not powerful enough to distinguish between these two explanations, but we should be able to sort this out with the help of the Cherenkov Telescopic Observatory, which is being built in the Canary Islands and Chile and is expected to begin observing in 2026.
“In some ways, there's a 50 percent chance that we could detect dark matter in huge amounts, but we'd need a new telescope to figure it out,” Silk says. If OGE due to dark matterthis will provide us with a better opportunity to understand this strange and mysterious material that holds the cosmos together.
Topics:
