Triangulum II: A Dark Matter Galaxy?

In Dark Matter by Brian Koberlein3 Comments

Dark matter is far more common in the universe than regular matter. Since it interacts gravitationally like regular matter, there has been speculation that some galaxies may contain lots of dark matter, but very little visible matter. These “dark galaxies” wouldn’t be particularly bright, but they would be distinguished by their strong gravity. We’ve found some evidence of dark matter galaxies before, and now a new paper proposes that Triangulum II could be a dark galaxy.

Triangulum II is a small galaxy on the edge of our Milky Way. It only contains about 1,000 stars, which makes it one of the smallest known galaxies. Recently a team of astronomers measured the speed of six stars in this galaxy, and found they were moving incredibly fast. So fast that if the stars constitute a majority of the galaxy’s mass these stars are escaping the galaxy (a process known as evaporation). It would be very unusual for all 6 stars to be evaporating from the galaxy at the same time unless it was being ripped apart by tidal forces. But that doesn’t seem to be the case.

The alternative is that these stars are still gravitationally bound by the galaxy, but that would mean the mass of the galaxy would need to be much larger than the 1,000 stars. From the speeds of these stars, they found it’s mass would need to be about 1.6 million solar masses. If that’s the case, then Triangulum II is the most dark-matter dominated galaxy we know by far.

What’s exciting about this discovery is that Triangulum II is a quiet galaxy where stars aren’t likely to be forming. This means we might be able to observe the effects of dark matter interactions. One idea for dark matter is that dark matter particles might collide with each other and produce gamma rays as a result. If we observe such gamma rays in Triangulum II, we can be pretty certain that they are the result of dark matter interactions rather than star production or other processes.

Paper: Evan N. Kirby, et al. Triangulum II: Possibly a Very Dense Ultra-Faint Dwarf Galaxy.  The Astrophysical Journal Letters, Volume 814 (2015).


  1. Just a question: How do we know that there ins’t a black hole in the center of those 1000 stars with a mass of 1.6 million solar masses?

  2. Another question – what makes Triangulum II a galaxy rather than a globular cluster or a dark globular cluster? I see by Wikipedia articles that there seems to be a degree of arbitrariness about the definitions of dwarf galaxies and clusters. There might just be a series of posts you could do about definitions and classifications.

    Thanks for all the great posts! I’m learning a lot.

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