Mysterious X-Ray Signals Have Astronomers Excited About Dark Matter

Dark matter is one of the biggest mysteries in our universe. We think it makes up about 85 percent of all matter, but because it doesn’t emit or absorb light like ordinary matter, we can’t see or measure it. But now, astronomers might (emphasis: might) have found traces of this enigmatic matter: a mysterious X-ray signal seen in a study of galaxy clusters. NASA reports that the signal could be from the decay of a type of particle astronomers have considered as a candidate for dark matter.

The potentially exciting new signal was found in a detailed study of the Perseus galaxy cluster by both NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton Observatory. Both detected a spike of intensity at an unexpected wavelength of X-ray light. But the spike turned out not to be unique to the Perseus galaxy cluster. XMM-Newton found the same emission line in a combined study of 73 other galaxy clusters.

One possibility is that the emission line is evidence of decaying sterile neutrinos.

Sterile neutrinos are a hypothetical type of neutrino that doesn’t interact through weak force; they’re predicted to interact with “normal” matter only through gravity. And while initial models found sterile neutrinos didn’t match predictions based on cosmological observations, a study published earlier this year came to a different conclusion. The lead author, Kevork Abazajian of the University of California, Irvine, suggested that a sterile neutrino with a mass of seven kilo-electron-volts could in fact be a candidate for dark matter.

In models, this hypothetical neutrino explained the latest X-ray data and answered some long-standing questions in galaxy structure and formation.

“We know that the dark matter explanation is a long shot, but the pay-off would be huge if we're right,” said Esra Bulbul of the Harvard-Smithsonian Center for Astrophysics in Cambridge. “So we're going to keep testing this interpretation and see where it takes us.”

“We have a lot of work to do before we can claim, with any confidence, that we’ve found sterile neutrinos,” added Maxim Markevitch from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But just the possibility of finding them has us very excited.”

There are a few reasons astronomers are being cautious in getting too excited over having potentially found traces of dark matter. For one, the emission line Chandra and XMM-Newton observed is right at the edge of both observatories’ sensitivity, meaning it might not be a real line. If the emission line turns out to be real, and even if this line is determined to be real, there may be other explanations.

It’s unlikely, but normal matter could produce the observed signal, though if this were the case it would impact what we know about the physical characteristics of galaxy clusters and atomic physics in hot gases. And even if the emission line turns out to be real and rooted in sterile neutrinos, there’s still the possibility that these hypothetical particles aren’t the material that makes up dark matter.

All this is to say there’s a lot more work to be done. “Our next step is to combine data from Chandra and JAXA’s Suzaku mission for a large number of galaxy clusters to see if we find the same X-ray signal,” said Adam Foster, also from the Harvard-Smithsonian Center for Astrophysics. “There are lots of ideas out there about what these data could represent. We may not know for certain until [new telescope] Astro-H launches, with a new type of X-ray detector that will be able to measure the line with more precision than currently possible.”

Though it’s by no means confirmed, the community of astrophysicists is excited by the discovery of this potential dark matter signal. Bulbul’s team’s paper has already been cited more than 50 times, and a second paper from a team led by Alexey Boyarsky of Leiden University in the Netherlands reports finding the same emission line in XMM-Newton observations of the galaxy M31, offering compelling evidence that the observed emission line is in fact real.

A little more time and research into this X-ray emission might go a long way in explaining one of the Universe’s biggest mysteries.