A satellite galaxy of the Milky Way seems to possess the largest density of dark matter in the local universe. The object, called Triangulum II, is made up of only 1,000 stars and is located 117,000 light-years from the center of the Milky Way.
Astronomers studied the velocity of six stars near the galaxy center, a feature dictated by the gravitational forces exerted on them. After using this to calculate the size of the force, the team was able to estimate that the mass of the small galaxy was the equivalent of 1.6 million solar masses, significantly larger than what one would expect from a galaxy housing just 1,000 stars.
The excess mass is likely due to dark matter, a hypothetical type of matter which interacts gravitationally with normal matter, helping keep galaxies together. Because it does not absorb or reflect light, it cannot be seen using telescopes. Galaxies tend to have 10 times more dark matter particles than normal matter, but in Triangulum II dark matter is about 3,600 times more abundant.
"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," Professor Evan Kirby said in a statement. "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."
If the estimate is confirmed, Triangulum II could be a perfect place to observe indirect evidence of dark matter. When dark matter annihilates itself, it emits powerful gamma rays. Often those signals are drowned out by the rest of the light emitted by galaxies, but since Triangulum II has only a small number of stars our telescopes should be able to detect the gamma signals from dark matter.
A group of researchers from the University of Strasbourg in France, have measured the stars on the edge of the galaxy and found that those stars are moving faster than the central group. If that were to be the case, it would indicate that the small galaxy is being pulled apart by the Milky Way, and that the dark matter estimate is not as accurate.
"My next steps are to make measurements to confirm that other group's findings," Kirby says. "If it turns out that those outer stars aren't actually moving faster than the inner ones, then the galaxy could be in what's called dynamic equilibrium. That would make it the most excellent candidate for detecting dark matter with gamma rays."
The paper was published in the Astrophysics Journal Letters.
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