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Emission Impossible? Is Dark Matter Behind the Hazy Radiation at the Milky Way's Center?

Some unknown astrophysical process, perhaps related to dark matter, may be at work



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.

When Fermi's first year of data went public in August, Dobler, Finkbeiner and their colleagues set to finding out whether the gamma-ray map of the galaxy indeed featured a haze akin to that seen by WMAP in the microwave. Sure enough, it did. "I had originally thought that it would be difficult to peel away all of the other types of things that make gamma rays in the galaxy, but I was kind of shocked," Dobler says. "It's almost jumping right out at you."

In the new paper Dobler and his colleagues describe the Fermi gamma-ray haze and make the claim that it confirms the synchrotron origin of the WMAP microwave haze. And as with the microwave haze, the authors argue that the electrons responsible for the gamma-ray haze appear to originate from an unknown astrophysical process.

Sean Carroll, a theoretical physicist at the California Institute of Technology who did not contribute to this haze research, calls the astrophysical claim that the hazes spied by Fermi and WMAP point to a population of high-energy electrons swirling around the inner galaxy "very reasonable." If the claim is true, he adds, "then the question is, 'Where do these electrons come from?' And one very plausible origin would be dark matter particles either decaying or annihilating into each other and creating a spray of particles."

Finkbeiner points out that he and his colleagues do not speculate in the new paper on a potential dark matter source for the haze-forming electrons. But both he and Dobler acknowledge moving in that direction. "We are absolutely in the process of exploring the Fermi haze in the context of dark matter physics," Dobler says.

But the implications of the gamma-ray and microwave hazes may remain open to challenges that the hazes themselves are as significant as they appear. "Everyone recognizes that it's a very tricky thing to look at all this messy radiation coming from the center of the galaxy and really to say that we've removed all the stuff we understand and are left with something we don't," Carroll notes.

And indeed, some researchers are not yet convinced that the hazes represent anything out of the ordinary. Astrophysicist Charles Bennett of Johns Hopkins University, principal investigator on the WMAP mission, says that his team has not viewed the haze as an excess of emission from the galactic center but as part of the natural astrophysical variation of synchrotron radiation. He points to a recent paper that seeks to explain the electrons by conventional astrophysical mechanisms, including massive supernovae, that do not require the influence of dark matter. "The observations, so far, can be explained either way," Bennett says, noting that the predictions of each model need to be checked against one another. "In the meantime, there is no particular evidence favoring what might be viewed as the more exotic explanation—dark matter decay."

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