Gamma rays offer mixed messages on identity of dark matter

A stream of high-energy gamma rays from the heart of the Milky Way is teasing scientists about the identity of the universe’s invisible matter.

The results of one study add to growing evidence that particles of dark matter, which collectively have more than five times as much mass as all the visible matter in the universe, are slamming into each other in the galactic center and emitting gamma rays. But another study of small, supposedly dark matter–rich galaxies finds no such gamma ray signal. The clashing results are the latest wrinkle in the long quest to discover the fundamental units of dark matter.

For decades scientists have known that there is an invisible component of matter in the universe that barely interacts with light and ordinary matter. So far dark matter has revealed itself only through its gravitational attraction, which influences the rate at which galaxies spin and the motion of galaxies within clusters. But some theorists predict that particles of dark matter should interact with each other and produce a detectable signal. In seeking that signature, scientists have pointed telescopes toward regions that should be rich in dark matter, such as the center of the Milky Way galaxy, about 25,000 light-years away.

In 2009, astrophysicists Lisa Goodenough and Dan Hooper pored over measurements from NASA’s Fermi Gamma-ray Space Telescope and found an unexpectedly high intensity of gamma rays, the most energetic form of electromagnetic radiation, emanating from the galactic center. Goodenough and Hooper suggested that the gamma radiation is the signature of weakly interacting massive particles, or WIMPs —proposed dark matter particles that would collide to produce electrons, positrons (the antimatter partner of electrons) and gamma rays.

Today most experts agree that there is a surprising amount of gamma radiation from the galactic center; scientists involved with the Fermi telescope endorsed that conclusion for the first time on October 22 at the Fifth International Fermi Symposium in Nagoya, Japan. Now the challenge is to prove that the gamma rays come from dark matter.

Kevork Abazajian, a cosmologist at the University of California, Irvine, looked for additional evidence of dark matter within the gamma ray signal. Theorists have predicted that WIMP collisions shouldn’t be the only way for dark matter to generate gamma rays. In another route, the electrons and positrons produced in the collisions could transfer some of their energy to starlight, which would bump up into the gamma ray spectrum. Abazajian and colleagues studied the intensity and energy of the Fermi telescope’s gamma ray data and found that a subset of the gamma rays appeared to have been produced by the enhanced-starlight process. The researchers reported their results October 23 at the Fermi symposium and October 22 on arXiv.org.

Abazajian says that dark matter is the best explanation for what his team is seeing, but it’s not the only possible explanation. The galactic center is a chaotic place that may contain rapidly spinning stellar corpses called pulsars and other high-energy objects that generate gamma rays. “Unfortunately, the place that has the most dark matter nearby is also the most complicated with everything else,” says Brandon Anderson, an astrophysicist and Fermi telescope collaborator at Stockholm University.

Anderson presented another complication for the dark matter argument October 24 at the Fermi symposium. He and his colleagues studied five years of Fermi telescope observations of small, dim galaxies that orbit the Milky Way. Unlike the galactic center, these dwarf galaxies consist almost entirely of stars and dark matter. If colliding WIMPs spew gamma rays in the galactic center, Anderson says, then the particles should do the same in dwarf galaxies. Yet Anderson’s team saw no signs of gamma rays from 25 dwarf galaxies. “We saw exactly what we’d expect when looking at a place without dark matter,” he says.

Marc Kamionkowski, a theoretical physicist at Johns Hopkins University who was not involved in the research, says there is still some wiggle room for dark matter despite the dwarf galaxy results. “It’s not a dagger in the heart of the dark matter explanation,” he says. Anderson agrees, but he says the study rules out the most popular range of masses for dark matter particles, unless those particles interact with each other much differently than theorists have predicted.