A Dead Galaxy at the Edge of the Milky Way Is a Dark Matter Bonanza

Astronomers at Caltech may have stumbled up the highest dark matter concentration in any known galaxy—and it happens to be just next door.

At first glance, there isn't all that much to Triangulum II. With around 1,000 stars calling it home, the dwarf galaxy is a fleck of dust compared to our Milky Way, which is thought to contain some 200 to 400 billion stars. That's fine though: Triangulum II is a "dead" galaxy. It's long-since stopped producing new stars and soon enough what's left will blink away.

Triangulum II might not be such an empty place, however. As described in a paper posted to the arXiv pre-print server and set for publication in the Astrophysical Journal Letters, the galaxy is seemingly overloaded with dark matter. This is so much so that the ratio of dark matter to regular matter within Triangulum II might prove to be the highest ever seen, making it an excellent quarry for directly detecting the stuff—a frustratingly elusive astrophysics milestone.

Determining how much dark matter in a galaxy is simple enough in theory. It's just a comparison between all of the stuff you can see and how much mass that should add up to, and the actual observed mass of the galaxy. The difference can be assumed to be dark matter.

To calculate their estimates, the Caltech team used the Low Resolution Imaging Spectrometer instrument at Hawaii's Keck Observatory. They were able to come up with a mass-to-light ratio—galactic mass compared to visible matter—of around 3500 (compared to 100 for the universe as a whole). Which is a lot of dark matter.

The claim that Triangulum II has the highest mass-to-light ratio of any known galaxy is dependent on it being in a state of dynamic equilibrium—that is, there isn't some huge external force mucking things up. So, we're going on a pretty big assumption here.

"It is unclear whether Tri II is in dynamical equilibrium," the Caltech group writes. "With a total luminosity of only 450 L⊙ [the unit of solar luminosity], the galaxy has very few stars available to measure its shape very precisely. Even fewer stars are available for spectroscopy."

Another group of astronomers, this one at the University of Strasbourg in France, has been measuring the velocities of stars around Tri II, finding that the further away from the galaxy's center they are, the faster they're moving. This is the opposite of what would be expected from a galaxy in equilibrium. The gravity of the Milky Way might be pulling it apart.

"My next steps are to make measurements to confirm that other group's findings," offers astronomer Evan Kirby, the study's lead author, in a statement. "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."