Artist's rendering of a gamma ray burst. Image: ESO/A. Roquette
When it comes to dark matter, the substance that makes up about 27 percent of the material in the universe, scientists have a better understanding of what it isn’t than of what it is. But scientists are slowly unraveling the mystery, bit by bit.
A new paper pulling together data from a gamma ray study is proverbially shedding light on dark matter. Researchers have developed maps of the center of our galaxy that shows a high production rate of gamma rays, a result consistent with the presence of some forms of dark matter.
The center of our galaxy is teeming with gamma rays, which originate from interactions between binary systems, isolated pulsars, supernova remnants, and particles colliding with interstellar gas. It’s in this active region that astronomers expect to find the highest density of dark matter, which would make itself known by imparting some gravitational effect on visible matter; dark matter helps bind matter into things like galaxies.
One leading candidate for the building block of dark matter is what's known as a Weakly Interacting Massive Particle, collectively known as WIMPs. When WIMPs collide, they either annihilate one another or produce a new particle that quickly decays. Either event produces gamma rays, which the Fermi Large Area Telescope (LAT) is designed to gather and measure.
The map at left shows gamma ray emissions from the galactic center, with known pulsars labeled. The map at right shows known gamma ray sources removed, leaving potential evidence of dark matter. Image: T. Linden, Univ. of Chicago
To create maps of the center of the galaxy, the team of researchers working with the Fermi LAT subtracted all known gamma ray sources from LAT observations of the galactic center. What they were left with was a patch of highly energetic gamma ray emission remains roughly one billion times more energetic than visible light, and extending 5,000 light-years from the galactic center. They're cannot fully explain the source of these leftover gamma rays, but suspect they’re related to the existence of dark matter.
"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 Dan Hooper, an astrophysicist at Fermilab who is a lead author on the new paper. "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.”
“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," said study co-author Tracy Slatyer, a theoretical physicist from the Massachusetts Institute of Technology.
There’s a lot more work that needs to be done before astronomers can confirm that what they’re seeing in this data in indeed indicative of dark matter. Part of the problem is that other gamma ray sources can produce strong signals that interfere with observations. But dwarf galaxies might hold the answer: According to NASA's Goddard Space Flight Center, dwarf galaxies orbiting the Milky Way tend to lack other types of gamma ray emitters, and have a large amounts of dark matter for their size.
If it does turn out that these observations are evidence of dark matter, this could turn out to be a major moment in astronomy. As Slatyer explained, “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.”