A new study of gamma-ray light from the center of our galaxy provides the strongest evidence so far that some gamma-ray emissions may originate from dark matter.
Scientists developed new maps, working with data from NASA’s Fermi Gamma-ray Space Telescope, showing that the galactic center produces more high-energy gamma rays than can be explained by known sources. The surplus emission is consistent with some forms of dark matter, an unknown substance making up most of the material universe.
“The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it. The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models,”
said Fermilab astrophysicist Dan Hooper, lead author of the study.
Galactic Center Dark Matter
The center of our galaxy is rife with gamma-ray sources. They range from interacting binary and triple star systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas.
The galactic center is also where astronomers anticipate finding the galaxy’s highest density of dark matter. Dark matter affects normal matter and radiation only by its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.
Dark matter’s true nature is unknown, but Weakly Interacting Massive Particles (WIMPs), are a leading class of candidates.
Theorists have imagined a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays, which are the most energetic kind of light, at energies within the detection range of Fermi’s Large Area Telescope (LAT).
Dark Matter Annihilation
After carefully subtracting all known gamma-ray sources from Large Area Telescope observations of the galactic center, leftover emission remains.
This extra amount seems to be most abundant at energies between 1 and 3 billion electron volts (GeV). This is about one billion times larger than that of visible light. The excess amount also fans out more than 5,000 light-years from the galactic center.
Hooper and his team conclude that annihilations of dark matter particles with a mass between 31 and 40 GeV provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center, and its overall brightness.
The researchers say that these features are difficult to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize.
“Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider (LHC), so if this is dark matter, we’re already learning about its interactions from the lack of detection so far. This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time,”
said co-author Tracy Slatyer of MIT.
Validation Still Needed
Although the vast amount of dark matter expected at the galactic center should produce a strong signal, competition from many other gamma-ray sources complicates any case for detection. However, turning the problem on its head provides another way to attack it. Instead of looking at the largest nearby collection of dark matter, look where the signal has fewer challenges.
“Our case is very much a process-of-elimination argument. We made a list, scratched off things that didn’t work, and ended up with dark matter,”
said co-author Douglas Finkbeiner.
“This study is an example of innovative techniques applied to Fermi data by the science community. The Fermi LAT Collaboration continues to examine the extraordinarily complex central region of the galaxy, but until this study is complete we can neither confirm nor refute this interesting analysis,”
said Peter Michelson, a professor of physics at Stanford University in California.
Dwarf Galaxy Confirmations
Dwarf galaxies orbiting the Milky Way do not have other types of gamma-ray emitters and possess large quantities of dark matter for their size. They actually are the most dark-matter-dominated sources known.
But since they are located much farther away and contain much less total dark matter than the center of the Milky Way, dwarf galaxies produce a much weaker signal and require many years of observations to establish a secure detection.
For the past four years, the LAT team has been probing dwarf galaxies for hints of dark matter. Published results from these studies have established rigorous limits on the mass ranges and interaction rates for many proposed WIMPs, even eliminating some models.
“There’s about a one-in-12 chance that what we’re seeing in the dwarf galaxies is not even a signal at all, just a fluctuation in the gamma-ray background,”
explained Elliott Bloom. If it’s valid, the signal should grow stronger as Fermi acquires more years of observations and wide-field astronomical surveys discover new dwarfs.
“If we ultimately see a significant signal. it could be a very strong confirmation of the dark matter signal claimed in the galactic center,”
Reference: Tansu Daylan,Douglas P. Finkbeiner, Dan Hooper, Tim Linden, Stephen K. N. Portillo, Nicholas L. Rodd, and Tracy R. Slatyer. The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter (pdf). FERMILAB-PUB-14-032-A, MIT-CTP 4533