Ghost town in space lurking closer to home
Small galaxy may have highest concentration of dark matter ever found
At the edge of the Milky Way, there’s a small galaxy called Triangulum II. It has just 1,000 stars, compared to the 100 billion estimated in our own galaxy, and its days of star formation are over, leaving it “dead.” But Triangulum II may have a dark secret — one that makes it the most interesting ghost town in space.
The nearby neighbourhood may have the highest concentration of dark matter ever found within a galaxy.
In a paper published Tuesday in the Astrophysical Journal Letters, researchers suggest that the mysterious, unseen matter may be responsible for Triangulum II’s abysmally low star count.
Scientists are still trying to hone in on how little dark matter interacts with the rest of the universe. For now, the answer seems to be: perplexingly little, but enough to keep things interesting.
As always, researchers in this study are detecting dark matter based on its effects — and the absence of anything more likely to cause those effects.
“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,” study author and Caltech assistant professor of astronomy Evan Kirby said in a statement.
To measure the gravitational forces influencing the inner workings of the galaxy — in other words, telltale signs of dark matter — Kirby and his colleagues had to rely on just six visible stars. The rest were all too dim.
“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,” Kirby said.
It’s possible that the strange galaxy isn’t as massive as these measurements suggest, which would negate the need for dark matter as an explanation. Another research group has suggested that the tiny galaxy is being torn apart by the Milky Way, which would be evidenced by stars on the edge of the galaxy moving faster than those in the middle and could muddle the kind of analysis Kirby and co. relied on. The researchers involved in the latest study are investigating this possibility, but they hope to show that Triangulum II really is full of dark matter.
If it is, the galaxy may be our bestever candidate for trying to detect the gamma rays that certain particles of dark matter produce when they interact with one another. It’s usually difficult to pick up these gamma rays in all the noise of space, but Triangulum II is so dead that we could probably manage to get a good look.