Gamma rays hint at dark matter in our galaxy

The coming decade will be the decade of dark matter, some scientists say, as efforts to detect the mysterious stuff will either pay off or rule out the most promising hypothesis about what it is. But astronomers may have already detected signs of dark matter in the heart of our own Milky Way galaxy, a pair of astrophysicists now says.

Data from NASA’s space-borne Fermi Gamma-ray Space Telescope reveal an excess of gamma rays coming from the galactic center; they could be produced as particles of dark matter annihilate one another, Kevork Abazajian and Manoj Kaplinghat of the University of California, Irvine, report in a paper posted to the arXiv preprint server.

“There’s definitely some source there, and it fits with the dark matter interpretation,” Abazajian said. But other researchers say the excess could be an artifact of the way Abazajian and Kaplinghat model the gamma-ray flux, or it could originate from more mundane sources.

Astronomers have ample evidence that dark matter provides most of the gravity that keeps stars from flying out of the galaxies. And cosmologists have shown that it makes up 85 percent of all matter in the universe. But physicists don’t know what dark matter is.

The leading hypothesis is that dark matter could be made up of weakly interacting massive particles, or WIMPs, which are predicted by some theories. WIMPs would be massive enough to produce lots of gravity but would otherwise interact with ordinary matter only very weakly. Each galaxy would form within a vast cloud of WIMPs.

Physicists are searching for WIMPs in several ways. Some are trying to spot them using exquisitely sensitive underground detectors. Others hope to produce WIMPs at the world’s largest atom smasher, the Large Hadron Collider in Switzerland. WIMPs might also annihilate one another when they collide to produce ordinary particles such as gamma rays, and astrophysicists are combing the heavens for signs of such annihilations.

Abazajian and Kaplinghat say the more than 400 researchers working with the Fermi satellite may have already found that evidence.

The two theorists analyzed data collected between August 2008 and June 2012, focusing on a 7-degree-by-7-degree patch of sky around the galactic center. For each of four energy ranges, they mapped the emission across the sky. They fit each map with a “baseline model” that included 17 pointlike sources of gamma rays that Fermi had already found in that area, a “diffuse” background that accounts for the general emission from the galactic center, and a spatially uniform background.

They then fit the data with a model that included a contribution from dark matter annihilations, including theoretical estimates of the dark matter’s distribution and how the particle annihilations produce gamma rays. Adding the dark matter annihilations greatly improved the fit, they found, suggesting that there is an excess of gamma rays that come from dark matter.

Other researchers, including Daniel Hooper of Fermi National Accelerator Laboratory in Batavia have made similar claims. In fact, Abazajian had previously argued against that interpretation. But the new analysis shows that the dark-matter hypothesis fits the data in three key ways, Abazajian says: It has the right energy distribution, the right spatial distribution and the right intensity.

When I saw that I was like, ‘Holy cow!’” he said. Abazajian cautioned, however, that the gamma rays could emanate from a less exotic source, such as previously undetected pulsars.

They might also be explained in an even easier way, said Stefano Profumo, a theoretical astrophysicist at the University of California, Santa Cruz, and a member of the Fermi-satellite team.

Abazajian and Hooper’s analyses depend critically on the model of the diffuse galactic background, Profumo says. That model had been derived to describe a much bigger area around the galactic center, he said, and is “completely blind to the details at the galactic center.” So its use the fits to the data could produce misleading results, he cautioned. Still, Profumo agrees that the galactic center is a prime place to look for evidence of dark matter.