Revealing the Galaxy’s Dark Side

Excess of gamma rays at Milky Way’s center may indicate universe’s missing mass.

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By Ron Cowen, Science News

For years, most claims that scientists had found evidence of dark matter, the ghostly material believed to account for more than 80 percent of the universe's mass, have seemed to dissolve into thin air. But a new claim of dark matter detection may have more than a dollop of cosmic credibility, scientists say.

Physicists Dan Hooper of the Fermi National Accelerator Laboratory in Batavia, Ill., and the University of Chicago and Lisa Goodenough of New York University base their findings, posted October 15 at arXiv.org, on an unexplained excess of energetic gamma rays emitted from the core of the galaxy. The gamma rays were recorded over the past two years with an instrument aboard NASA's Fermi Gamma-ray Space Telescope, launched in 2008.

Dark matter, like ordinary atomic matter, is expected to concentrate at the galaxy's center. That makes the Milky Way's crowded core one of the most promising places to look for signs of the dark stuff, theorists agree. It's also one of the most complex places to search, because the core is riddled with a variety of ordinary but poorly understood sources of gamma ray emission, notes Fermi scientist Steve Ritz of the University of California, Santa Cruz.

Hooper and Goodenough analyzed gamma rays recorded by Fermi from the innermost 175 light-years of the galaxy and found a sharply rising gamma-ray signal that peaked at energies between 2 billion and 4 billion electron volts, about a billion times the energy of visible light. Hooper asserts that the location and energy of the gamma rays can't easily be explained by run-of the-mill sources, such as ultradense, rapidly spinning stars called pulsars.

"In our paper, we discussed a number of astrophysical possibilities for the origin of the signal, including a population of pulsars, cosmic ray interactions and emission from our galaxy's supermassive black hole," notes Hooper. "And in the end, no combination of any astrophysical sources could give us the signal we're seeing," he adds. "Eventually we just got fed up and concluded there doesn't seem to be a way to explain the signal except for one thing—we tried dark matter and it fit beautifully without any special bells or whistles."

Astronomers require some kind of dark or missing matter to explain why galaxies and galaxy clusters don't fly apart, and how the universe evolved from the Big Bang to its present state. The gravitational glue provided by ordinary matter isn't nearly enough to keep the universe intact, or explain how the complex structure of the cosmos came to be. The density of dark matter inferred by Hooper and Goodenough is in the right ballpark to account for the missing material, the team says.

In a paper Hooper and Goodenough posted online last year at arXiv.org, based on only a year of Fermi data, the team was more circumspect. But with the second year of data, says Hooper, "this is the most confident I have ever been that something we were seeing in an experiment was a signal of dark matter."

In addition, the mass of the proposed dark matter particles that Hooper and Goodenough infer from the analysis is consistent with findings from two direct dark matter detection experiments—COGENT, located in the Soudan mine in northern Minnesota, and DAMA, in the underground Gran Sasso National Laboratory near Rome (SN: 8/28/10, p. 22).

Hooper says he's particularly excited by the apparent match with the COGENT and DAMA experiments, results he had been independently considering for several months. "You should have seen the look on my face when those numbers came out of my computer code. I thought, ‘No one is going to believe this.'…Either this is something or this is a remarkable coincidence. And I think this is something."

Physicist Neal Weiner of NYU says that just because the galactic center "is a tricky place to study, to be sure, that doesn't mean one can brush a signal like this aside. This feature has a dramatic cutoff in its spectrum and a rapid falloff as a function of radius. I don't know of a population of astrophysical objects that has that distribution."