What started as measuring the mass of a nearby dwarf galaxy, quickly shifted towards dark matter. Caltech’s Evan Kirby was taking a closer look at Triangulum II, a small galaxy at the edge of the Milky Way with just 1,000 stars. That’s a far cry from the 100 billion + stars in our Milky Way galaxy.

Observing Triangulum II wasn’t easy. “Only six of its stars were luminous enough to see with the Keck telescope,” said Kirby. To measure its mass, Kirby examined the velocity of these six stars. Armed with these measurements, Kirby could figure out the gravitational force exerted on the stars and come up with the mass of the galaxy.

Keck Observatory

Keck Observatory

What Kirby found made him say “wow.”

“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,” according to Kirby. “The ratio of dark matter to luminous matter is the highest of any galaxy we know.”

These findings put Triangulum II towards the top of the pack for trying to detect dark matter. Scientists have yet to directly observe signals from dark matter. They believe it exists due to its gravitational influence in galaxies, but observing it directly is a challenge.


A particular particle of dark matter known as supersymmetric WIMPs (weakly interacting massive particles) destroy each other when they collide. The byproduct of this collision is gamma rays. So just look for gamma rays, right? One problem. Dark matter isn’t the only thing emitting gamma rays. Pulsars and other galactic phenomenon make detecting dark matter gamma rays difficult.

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Triangulum II is an intriguing target because it’s a galaxy astronomers describe as “dead.” No new stars are formed, so gamma ray signals from dark matter particles should be visible (theoretically).

Are Kirby’s findings correct?

Kirby’s findings haven’t been 100% confirmed. Another group of researchers (from the University of Strasbourg in France) looked at the velocities of stars just outside Triangulum II. They found these stars were moving faster than the ones inside Triangulum II – which was unexpected. If these findings hold up, it will contradict Kirby’s mass estimate for the galaxy.

Kirby will take measurements of these outer stars to see if the other group’s findings are correct.

“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,” says Kirby.

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