The Darkness on the Edge of the Galaxy

I’d guess that when most people think of galaxies, they think of stars. After all, that’s the source of a galaxy’s beauty to our eyes, along with the gas and dust of spiral galaxies. On the other hand, we know that most of a galaxy’s mass is in the form of dark matter, and based on our understanding of how galaxies form, some astronomers have wondered: could there be galaxies with almost no stars? From a certain point of view, a galaxy can be thought of not as a collection of stars, but as something of a certain mass bound together by its own gravity. (As I discussed in an earlier post, a galaxy cluster can be thought of as a collection of dark matter and hot gas, with the galaxies themselves almost an afterthought to many astronomers and cosmologists!)

Segue 1 galaxy: a galaxy with almost no stars. Can you spot it?

So it was with excitement that I saw the discovery by astronomers from the Keck Observatory of a dark galaxy. Called Segue 1, this galaxy contains only about 1000 stars, but 600,000 times the mass of our Sun. In other words, the stars are only a fraction of 1% of the total mass of the galaxy, and there’s no substantial gas or dust to make up the difference. Because there are so few stars, it’s hard to even see that the galaxy is there! In fact, Segue 1 was first found 2 years ago: careful analysis and observation was needed just to get a sufficient view of the galaxy to characterize it.

Observation of small, faint galaxies like Segue 1 is hard and requires a lot of patience, but the implications are intriguing. We have a good idea of how much total dark matter there is in the universe, but  that’s not the same thing as knowing exactly where it is. The existence of dark galaxies gets us a little closer to creating a picture of how galaxies form, and the total distribution of dark matter throughout the cosmos.

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