Tons of Dark Matter Hide in Nearby Dwarf Galaxy

By measuring the mass of a nearby dwarf galaxy called Triangulum II, researchers may have found the highest concentration of dark matter in any known galaxy.
Tons of Dark Matter Hide in Nearby Dwarf Galaxy
A powerful collision of galaxy clusters has been captured by NASA's Hubble Space Telescope and Chandra X-ray Observatory. This clash of clusters provides striking evidence for dark matter and insight into its properties. NASA
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By measuring the mass of a nearby dwarf galaxy called Triangulum II, researchers may have found the highest concentration of dark matter in any known galaxy.

Although they outweigh particles of regular matter by more than a factor of 5, particles of dark matter are elusive. Their existence is inferred by their gravitational influence in galaxies, but no one has ever directly observed signals from dark matter.

Triangulum II is a small, faint galaxy at the edge of the Milky Way, made up of only about 1,000 stars. Evan Kirby, assistant professor of astronomy at Caltech, measured the mass of Triangulum II by examining the velocity of six stars whipping around the galaxy’s center.

“The galaxy is challenging to look at,” he says. “Only six of its stars were luminous enough to see with the Keck telescope.” By measuring these stars’s velocity, Kirby could infer the gravitational force exerted on the stars and thereby determine the mass of the galaxy.

“The total mass I measured was much, much greater than the mass of the total number of stars—implying that there’s a ton of densely packed dark matter contributing to the total mass,” Kirby says. “The ratio of dark matter to luminous matter is the highest of any galaxy we know. After I had made my measurements, I was just thinking—wow.”

Triangulum II could thus become a leading candidate for efforts to directly detect the signatures of dark matter. Certain particles of dark matter, called supersymmetric WIMPs (weakly interacting massive particles), will annihilate one another upon colliding and produce gamma rays that can then be detected from Earth.

While current theories predict that dark matter is producing gamma rays almost everywhere in the universe, detecting these particular signals among other galactic noises, like gamma rays emitted from pulsars, is a challenge.

Triangulum II, on the other hand, is a very quiet galaxy. It lacks the gas and other material necessary to form stars, so it isn’t forming new stars—astronomers call it “dead.” Any gamma ray signals coming from colliding dark matter particles would theoretically be clearly visible.

Lori Dajose
Lori Dajose
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