STRANGE HAZE: Sky maps obtained by NASA's Fermi Gamma-Ray Space Telescope show a peculiar haze when ordinary gamma-ray sources are subtracted. (The map shown here has not been source-filtered and is presented for illustration.)
NASA/DOE/International LAT Team

A haze of radiation at the heart of the Milky Way Galaxy that appears in sky maps taken by two spacecraft at two different wavelengths likely results from a population of high-energy electrons, according to a new analysis of gamma rays in the galaxy. Curiously, some researchers maintain, those electrons are not readily explained by known astrophysical processes—and work is under way to determine if dark matter particles might be responsible.

Dark matter is a hypothesized material that pervades the universe but does not interact with light in a way that we can perceive. Current estimates rate dark matter as being roughly five times as prevalent as ordinary matter—the atoms and molecules that make up the familiar physical world. To date, dark matter has been observed only indirectly via its gravitational effects, but its true nature remains a mystery.

A paper posted to the physics preprint Web site arXiv.org on October 26 and submitted to the Astrophysical Journal points to a possible signature of dark matter in the Milky Way, although the study's authors are careful to keep their observations empirical and table such speculation—for the moment, at least.

In 2003 Douglas Finkbeiner, an astronomer at the Harvard–Smithsonian Center for Astrophysics, noticed a diffuse haze toward the center of the galaxy in microwave data collected by NASA's Wilkinson Microwave Anisotropy Probe (WMAP). There are only a handful of processes that yield microwaves in the interstellar medium, Finkbeiner explains, and when he subtracted templates for those processes from the WMAP data, something curious remained. "If our model [for microwave production] were correct, we would have random noise left over," Finkbeiner says. "Instead, we see a pattern, an excess of microwaves in the inner galaxy."

He and his colleagues figured that the microwaves were synchrotron emission: photons emitted by electrons accelerated by the galaxy's magnetic field. But the energy spectrum of the electrons was not readily accounted for by conventional sources in the inner galaxy—for instance, electrons originating from supernovae explosions. So, a popular model for dark matter, in which the dark particles would annihilate each other on contact in a burst of observable particles, including electrons, seemed instead to fit the bill.

The high-energy electrons suspected as the progenitors of the WMAP haze should produce a similar haze in the gamma-ray regime, Finkbeiner and his colleagues predicted. "We would expect that those same electrons that are spiraling around the galactic magnetic field will once in a while hit a photon coming from a star or something like that," says Gregory Dobler, a postdoctoral researcher at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, and lead author of the new arXiv study. Such electrons can bump up optical or infrared photons to gamma-ray energies, Dobler notes, where they could be detected by NASA's Fermi Gamma-Ray Space Telescope.