Dark matter: when two massive things met

Millions of years, ago, astronomers say, a ball of dark matter with the mass of about five million suns appears to have ploughed through a long stream of stars, disrupting their flow in a way that can only be explained by an interaction with something unknown, but massive.

“We see stars pulled out of this stream by a dark object,” Ana Bonaca, an astronomer at Harvard University’s Centre for Astrophysics, in Massachusetts, US reported this week at a meeting of the American Astronomical Society in St Louis, Missouri. 

“We think this is, plausibly, a clump of dark matter in the Milky Way halo.” 

Star streams are remnants of giant star clusters or dwarf galaxies that have been pulled apart and stretched by the gravity of other objects, such as the Milky Way itself. The one Bonaca’s team chose for study, known as GD-1, is both well known, and long. 

“The longer the stream, the more time it would have had to interact with anything else,” she says.

Dark matter, on the other hand, is an unknown form of matter, to date detected only on the basis of its gravitational effects. 

We know it exists in and around the Milky Way, Bonaca says, because it appears to have vastly more mass than can be accounted for by stars, dust and gas, with much of it falling within a halo that surrounds the galaxy like an invisible orb. “Something like 90% of the mass of the galaxy is invisible,” she says.

Theories of dark matter vary, with some predicting that it is formed of large particles, the size of protons, which might have a tendency to form massive clumps. Other theories suggest it might be formed of much smaller particles — millions of times smaller — that would form fewer clumps, or maybe none at all.

That, she says, makes it important to look for ways in which dark matter clumps might interact with other stellar objects, such as the GD-1 star stream, in order to see how many (if any) such clumps exist. 

It’s not a new idea, but getting good results required not only detailed sky maps of the GD-1 stream, but also the latest data from the European Space Agency’s Gaia space telescope, which can reveal not only the locations of stars in the stream, but also how groups of them are moving in tandem. 

Based on this, she says, GD-1 appears to have two gaps, plus a trail of stars that are being pulled off at a slightly different angle, like a split tail on a comet.

One gap is probably created by the gravity of the Milky Way. But the other, plus the split tail, can’t be explained by gravitational interaction with any known object. 

“So we have evidence that something happened to this stream,” Bonaca says. “It was perturbed by a massive object orbiting in the Milky Way halo.”

Computer modelling, she adds, suggests that this object probably carried about five million solar masses, and encountered through the GD-1 stream about half a billion years ago. 

“It pulled stars out of the stream,” she says. 

Like any scientist, she’s cautious to generalise her results too strongly, especially because so far all she’s found is evidence of a single star stream being disrupted by a single massive object. 

“Samples of one are always hard,” she says. “It’s hard to argue that it’s not just a random black hole passing through.”

Still, she says, her study is proof that star streams like GD-1 can be used to find signs of perturbation by invisible objects, including dark matter. 

“We are looking at other streams,” she adds.

And if dark matter clumps that massive do indeed exist, she says, it’s important information for theoretical physicists, because “that would rule out many dark-matter models”.

Related reading: Closing in on dark matter