By measuring the mass of a nearby dwarf galaxy called Triangulum II, an astronomer in the US has detected what could be the highest concentration of dark matter in any known galaxy. With a relatively small number of stars, this small, faint galaxy is not the easiest to investigate, but there was no mistaking how unusual its light-to-dark ratio is.
"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," says Evan Kirby from the California Institute of Technology. "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."
Discovered earlier this year at the edge of our Milky Way galaxy, the Triangulum II galaxy has about the same luminosity as the faintest galaxy in the known Universe, called Segue 1, and contains just 1,000 stars. To put that in perspective, the Milky Way has around 100 BILLION stars.
The problem with having so few stars is that it makes finding big, bright stars difficult, and these are the only ones we’re able to reliably detect with Earth-based telescopes. Fortunately, a handful of stars is all you need to measure the gravitational force being exerted on them - you just need to see them clear enough to measure their velocity. Once you’ve used the velocity of the stars to figure out the gravitational force they’re dealing with, you can determine the mass of the galaxy.
Kirby found six stars zooming around the centre of Triangulum II that did the trick. "The galaxy is challenging to look at," he says. "Only six of its stars were luminous enough to see with the Keck telescope."
To us, the known Universe appears to consist of planets, stars, galaxies, and all kinds of dust, rock, and gas, but in reality, all that visible - or 'normal' - matter makes up just 4.9 percent of it. Dark matter outnumbers particles of normal matter by more than a factor of 10, making up 26.8 percent of the Universe, while dark energy takes care of the remaining 68.3 percent.
But just because dark matter and dark energy make up the majority of our Universe doesn’t mean we can see them - that’s why they’re called ‘dark’. The only way we know how to detect dark matter and dark energy is by indirectly measuring their presence via their gravitational influence on various components of a galaxy.
So far, no one has managed to directly measure or detect a dark matter signal, but the discovery that Triangulum II has the highest concentration of dark matter of any known galaxy could be key to figuring out how to do it.
Special types of dark matter particles called WIMPs are known to annihilate each other when they collide, and this interaction produces gamma rays that can be detected using Earth-based equipment. These gamma rays are about as close as we can get to directly detecting dark matter, but even though it’s thought that they’re produced just about anywhere in the Universe, they’re constantly being drowned out by other types of gamma rays, such as those emitted by pulsars - dead relics of massive stars.
That’s why Triangulum II is such an amazing find: not only does it appear to have the highest known concentration of dark matter, the fact that it’s so small and has so few stars, means that competing signals don’t appear to be a problem. It’s so quiet, astronomers refer to it as 'dead', which means it should be relatively easy to detect dark matter-produced gamma rays. Fingers crossed we find something great in the darkness.
Kirby’s findings have been published in Astrophysical Journal Letters.