A Stellar Stream in the Milky Way Provides Evidence of Dark Substructure

Scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City have observed what may be evidence of dark matter interfering with a stellar stream in the Milky Way galaxy.

“We know that 90 percent of the mass in our universe is invisible. We don’t know what it is, but we’re curious,” says Ana Bonaca, an ITC Fellow at the CfA, and lead author of the study. “Stellar streams, which is what we’re studying here, tell us the story of our galaxy. They’re so long and thin, they’re sensitive to the tiniest disturbances as they orbit through the galaxy. Our findings are that … in action.”

The researchers presented their results July 23 in the Astrophysical Journal. The team consisted of Bonaca, CCA Flatiron Research Fellow Adrian Price-Whelan, CfA scientist Charlie Conroy, and David Hogg, a CCA group leader and an astrophysicist at the Center for Cosmology and Particle Physics at New York University.

The work began in April 2018 during an event at the CCA that brought together scientists to study the second data release of Gaia, a mission of the European Space Agency (ESA). The released data included a new view of GD-1, the longest and most visible thin stellar stream in the Milky Way. Typically, stars are distributed almost uniformly along such streams. Here, the scientists immediately noticed that some of the stars in the GD-1 stream were not behaving as expected.

“Stellar streams were thought to be more or less smooth in appearance,” says Price-Whelan, “but GD-1 has gaps or regions of lower density along the stream. Close to one of these gaps there is an offshoot of misaligned stars. So, first, we found something interesting that didn’t match what we expected to see, thanks to Gaia.”

Stellar streams are associations of stars that once belonged to a dwarf galaxy or a globular cluster but that were pulled out by the Milky Way’s tidal forces and stretched out into streams. In a standard picture, these streams are long, thin, and regular. The observed behavior in GD-1, however, could not be explained by tidal forces alone. Using numerical simulations, Bonaca and her collaborators showed that the observed gap and spur features could instead be the result of the stream encountering a dense, massive object.

“We considered a number of different objects as potential sources of perturbation, but none of them seemed to fit,” says Bonaca. “We looked at the orbits of all known satellites in the galaxy, but none [had] crossed paths with GD-1 recently. We also considered whether molecular clouds could have done the damage, because GD-1 crosses the Milky Way disk, but found they [were] not dense enough. There [was] no obvious culprit.”

With no known culprits, scientists have turned to more exotic explanations, and that’s big news for dark matter theorists. “One of the fundamental predictions of the dark-matter model is that there ought to be many concentrations or clusters of dark matter orbiting in the outskirts of our galaxy,” says Hogg. “This stream looks like it can be used to find those small clumps of dark matter. Ruling out all other possibilities, and actually detecting a small clump of dark matter, would be a huge clue for understanding the nature of this important component of the universe.”

While Gaia data was used to make initial observations, the team has since conducted follow-up observations with Hectochelle — a multiobject echelle spectrograph — at the MMT Observatory, located at the Fred Lawrence Whipple Observatory at Mt. Hopkins in Arizona. These new data will help in locating the dark substructure. In addition, Bonaca and other scientists have begun observing other stellar streams with unusual features.

“When something passes close to a stellar stream, it leaves evidence behind, and we can see that something happened there. Even if it’s dark matter. Even if it’s invisible,” says Bonaca. “And if it is a clump of dark matter, there should be many of them. So we’re setting out to search for such oddities in other streams, to find out for sure.”