The heart of our galaxy is oddly bright. Since 2009 astronomers have suggested that too much gamma-ray light is shining from the Milky Way’s core—more than all the known sources of light can account for. From the beginning scientists have suspected that they were seeing the long-sought signal of dark matter, the invisible form of mass thought to pervade the universe. But two recent studies offer more support for an alternate explanation: The gamma rays come from a group of spinning stars called pulsars that are just slightly too dim to see with current telescopes.
Part of the confusion stems from uncertainties about the gamma-ray signal, which shows up in data from NASA’s Fermi Gamma-Ray Space Telescope. Many different groups have analyzed Fermi’s publicly available data and claimed to see an unexplained excess of light, but the details of what they find and how they interpret it vary from group to group. Now, for the first time, the Fermi telescope team has confirmed the puzzling excess in a paper submitted to The Astrophysical Journal. The study offers the best description yet of the particularities of the extra light, such as its density and spread in space and its wavelength spectrum as well as all of the contaminating factors, such as systematic errors in the telescope and other sources of gamma-ray light that may muddy the signal. The team’s analysis stokes hopes that scientists may finally be close to making sense of the signal.
On the trail of dark matter
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The gamma-ray glow has long intrigued theorists who say it uncannily matches predictions for a particular explanation of dark matter. Dark matter must be all around us because the stars and galaxies feel its gravitational pull—but its makeup is unknown. One suggestion is that “weakly interacting massive particles,” or WIMPs, account for dark matter. These particles would be their own antimatter counterparts, and just as matter and antimatter destroy one another on contact, two WIMPs would annihilate if they collided. At the center of the Milky Way, where dark matter is thought to be extremely dense, WIMPs would often smash together and their explosions would likely give off gamma-ray light—just as Fermi sees.
But the light may have a more mundane origin. Pulsars are the remnants of once-large stars that have run out of fuel for nuclear fusion and collapsed. They spin at dizzying speeds—many make a full rotation every millisecond—and shine their light in condensed beams that rotate with them like a lighthouse. Pulsars are known to emit gamma-rays and could conceivably contribute to the surplus if there are enough of them hiding in the galactic center. Two recent studies that came out before the Fermi analysis support this scenario by finding that the extra gamma-ray light looks a bit more clumpy than smooth. Clumps would be expected if the light originated from individual objects—like pulsars—rather than from dark matter particles spread evenly through space. “Having a model based on point sources rather than smooth emission changes your statistics,” says Tracy Slatyer, a physicist at MIT. Together with her collaborators, Mariangela Lisanti and Samuel Lee of Princeton, and Ben Safdi and Wei Xue of MIT, she found a “striking” preference in the data for pulsarlike points of light. A separate study using different statistical methods, led by Christoph Weniger at the University of Amsterdam, came to the same conclusion. “Right now, I think millisecond pulsars are the best bet,” Weniger says. “Although everybody would like to find a dark matter signal, we have to be careful and not to jump to conclusions.”
Not everyone is persuaded, however. “I think these point source papers are very interesting, and I have considered their arguments carefully,” says Dan Hooper, an astrophysicist at the Fermi National Accelerator Laboratory who was one of the first to point out the gamma-ray surplus. “That being said, I am not convinced that point sources are responsible for the excess.” It seems just as likely, he says, that the evidence for clumpiness is a result of mistaken assumptions about other sources of gamma rays such as interactions between cosmic rays and the gas between stars. “At this point, I don't think the answer is clear.”
And one issue with the pulsar explanation is the question of why so many would be clustered in a sphere around the hub of the Milky Way. “It really looks like a different population that was formed in a different way” than familiar pulsars, Slatyer says. Astronomers Timothy Brandt and Bence Kocsisof the Institute for Advanced Study in Princeton have suggested that star clusters orbiting the Milky Way might have been disrupted by our galaxy’s gravity, causing them to spill out stars, including pulsars, into a spherical shell in the middle of the galaxy. “The nice thing about this explanation is that the model they use was developed for a different purpose,” Slatyer says, “and what they get actually lines up very well” with the galactic center signal.
The comprehensive information on the gamma-ray light in the new Fermi collaboration study should help clarify the situation. “I think quite highly of the new paper,” Hooper says, adding that it “fills us in about many of the details.” The Fermi team itself is agnostic about the source of the light. “We can conclude that there is an excess on top of the conventional gamma-ray emitters, and there’s certainly an indication that there is something new, but it’s too soon to conclude that there is a dark matter signal,” says Simona Murgia of the University of California, Irvine, one of the primary co-authors of the Fermi paper. As for pulsars, “myself, I would say they are equally plausible.”
Finding proof
The good news is that if pulsars are behind the excess, more powerful, telescopes in the future should be able to spot the too-faint spinning stars directly. Pulsars would be prime targets for next-generation radio telescopes such as MeerKAT under construction in South Africa and the Square Kilometer Array (SKA), which encompasses MeerKAT, set to become operational in southern Africa and Australia in 2020. “Should we fail to find them in the next five or ten years, a dark matter explanation becomes more likely again,” Weniger says. “This is pretty much a win–win situation. But we have to be patient.”
Meanwhile support for the dark matter explanation could appear even sooner. If WIMPs are responsible for the invisible matter, they might arise in the particle collisions taking place at the world’s largest atom smasher, the Large Hadron Collider (LHC), which was recently restarted at its highest energies yet. So-called direct detection searches are also looking for WIMPs in underground experiments aimed at catching the elusive particles in the rare act of interacting with regular matter. The fact that neither particle accelerators nor direct detectors have yet seen dark matter particles has already put strong constraints on the types of WIMPs that could exist. Astrophysicist Francesca Calore of the University of Amsterdam and her colleagues recently combined the theoretical constraints from all the various searches as well as data from the galactic center. They found many WIMP models are already excluded by the data but some remain plausible. “It came out that there are a few interesting regions that can be probed by the next rounds at the LHC, which can actually be tested in the next year,” Calore says.
Another check on the dark matter hypothesis comes from dwarf galaxies. After all, if WIMPs are annihilating at the center of the Milky Way, they must also be doing so at the cores of other galaxies. The signal would be too dim to see in neighboring large galaxies but should show up in “dwarf spheroidal” galaxies orbiting the Milky Way, which are thought to be extremely dense with dark matter. “But there’s no signal from dwarfs,” says Kevork Abazajian of U.C. Irvine, noting that only one of the roughly 30 known dwarfs shows a hint of gamma-ray excess. “The story seems to be that the dwarfs are dark and the galactic center is bright.” In fact, the lack of gamma rays in the dwarf galaxies seems to cast significant doubt on the dark matter explanation for the Milky Way glow, Abazajian and a collaborator found in a paper submitted last month to the preprint server arXiv.
If dwarf galaxies, particle accelerators and direct detection experiments continue to come up empty in future years, the plausibility of the WIMP explanation for dark matter—and for the Milky Way’s excess gamma rays—will get further stretched. The same goes for the search for pulsars in the galactic center. Soon these ideas should either be confirmed or disproved. “It’s always been my fear with this excess that we’d never find a confirming signal in any other dark matter channel but also no good astrophysical observations” such as pulsars, Slatyer says. “That’s a very frustrating situation to be in as a scientist.” Luckily that eventually, she says, is looking less and less likely.