14 DAY TRIAL // Observations of 10 dwarf galaxies orbiting the Milky Way, conducted using the NASA Fermi Gamma-ray Space Telescope, recently placed heavy limitations on the nature and energy range of theoretical particles thought to make up dark matter.
The stuff is believed to account for about 80 percent of the mass in the Universe, but it is invisible to existing detection methods, and is believed to interact with baryonic (normal) matter solely through gravity. Astrophysicists have some proposals concerning what dark matter is made out of.
One of the main proposals is that dark matter should produce faint gamma-ray signals and new types of subatomic particles. Identifying them would confirm the existence of the stuff. This is precisely why Fermi was used to search for the proposed signals. Its detectors are very sensitive at these wavelengths.
“One of the best places to look for these faint gamma-ray signals is in dwarf spheroidal galaxies, small satellites of our own Milky Way galaxy that we know possess large amounts of dark matter,” explains Jennifer Siegal-Gaskins, a post-doctoral fellow at the California Institute of Technology (Caltech).
“From an astrophysical perspective, these are downright boring systems, with little gas or star formation and no objects like pulsars or supernova remnants that emit gamma rays,” she adds.
The study Fermi conducted on the 10 dwarf galaxies did not reveal any signs of the proposed signals, but that in itself is an important discovery. Using its Large Area Telescope (LAT) instrument, the observatory was able to eliminate these particles as candidates to explain the nature of dark matter.
“In effect, the Fermi LAT analysis compresses the theoretical box where these particles can hide,” says Siegal-Gaskins, who also holds an appointment as a member of the Fermi LAT Collaboration.
At this point, most astrophysicists favor Weakly Interacting Massive Particles (WIMP) as the most likely constituents of dark matter. These particles are thought to interact rarely with anything else, and to be their own antimatter particles. A collision between two WIMP would annihilate both particles.
The interaction would also produce gamma-rays, the theory holds, and the LAT instrument was designed specifically to be extremely sensitive at these wavelengths. Over 2 years, it analyzed high-energy radiation – from 200 million to 100 billion electron volts – from the 10 dwarf galaxies.
When analyzing the data, researchers could not find any indicator of WIMP-produced gamma-rays.
“This treatment constitutes a significant step forward, and we hope that future studies will follow our example,” Stockholm University physics professor and study co-author, Jan Conrad, concludes, quoted by Daily Galaxy.