Dark matter is elusive. Dark matter particles significantly outnumber regular matter particles, experts said, but the only way to detect its existence is by inferring its gravitational effects on galaxies.
Through this method, a scientist at the California Institute of Technology (Caltech) found that a dwarf galaxy at the edge of the Milky Way contains the highest concentration of dark matter that experts have ever seen--albeit not literally.
Greater Than The Sum Of Its Parts
Caltech Astronomy Assistant Professor Evan Kirkby measured the mass of Triangulum II, a small and faint galaxy that is made up of only about 1,000 stars, by examining the velocity of six stars found in the galaxy's center. This small galaxy is 117,000 light-years away from our planet.
Kirkby said the galaxy's six stars were the only ones luminous enough to be seen through the Keck telescope. He then inferred the gravitational force on the stars and calculated the mass of Triangulum II.
"The ratio of dark matter to luminous matter is the highest of any galaxy we know," he explained. The total mass Kirkby measured was extremely greater than that of the sum of the galaxy's stars. It implies that a ton of dark matter, which is densely-packed, is contributing to the galaxy's total mass, he said.
Gamma Rays From A Dead Galaxy
The number of stars found in Triangulum II is relatively small compared to the 100 billion stars in the Milky Way Galaxy. Scientists said Triangulum II is considered a dead galaxy because it no longer produces new stars.
The faint galaxy was discovered earlier in 2015 and its luminosity is similar to that of the faintest galaxy in the universe known as Segue 1, scientists said.
Still, the Triangulum II might become a chief target in attempts to understand the signatures of dark matter. Kirkby and his colleagues said that specific dark matter particles called supersymmetric weakly interacting massive particles or WIMPs will destroy one another as soon as they collide. This collision will produce gamma rays that scientists on Earth can detect.
Existing theories estimate that dark matter is creating gamma rays in almost any part of the universe, and locating these signals among galactic noises, such as gamma rays from pulsars, is a challenge for scientists. However, because Triangulum II is a dead galaxy, any signals of gamma rays from dark matter particles that have collided would theoretically be visible, experts said.
Further Studies
Kirkby's calculations are still not definitively confirmed. Another group of researchers from France's University of Strasbourg also measured the velocities of the stars outside Triangulum II and discovered that they are whirling faster than the stars in the center of the small galaxy.
Scientists said this was the opposite of what was expected, and that this implied that Triangulum II is being pulled apart by the gravitational forces of the Milky Way galaxy.
"My next steps are to make measurements to confirm that other group's findings," said Kirkby. He said that if those outer stars are not really moving faster than the inner ones, then Triangulum II might be in a dynamic equilibrium.
"That would make it the most excellent candidate for detecting dark matter with gamma rays," he added.
Kirkby and his colleagues published their findings in The Astrophysical Journal Letters on Nov.17.
