The Milky Way hasn’t been kind to the Sagittarius dwarf galaxy. Located some 70,000 light-years away, the bundle of stars has been shredded and stretched into a filamentous stream by the gravity of the Milky Way. Now, scientists have mapped Sagittarius in exquisite detail, and they’ve used that map to provide a long-sought picture of the mysterious dark matter halo in which our Galaxy resides.
First spotted in 1994, Sagittarius is one of the Milky Way’s closest companions. Across the ages, gravitational forces have ripped it apart, scattering stars into a stream that now completely encircles the Milky Way. That makes Sagittarius a sensitive scale for measuring the distribution of mass in our Galaxy, which includes not just the visible disk of stars, but also an unseen halo of dark matter, thought to comprise up to 90% of the total mass.
In principle, researchers could monitor the orbits of nearby star clusters and galaxies and use the laws of physics to calculate how much matter is tugging on them. But their motion across the sky is too slow to help within human lifetimes. The Sagittarius stream, on the other hand, already embodies those motions. “It’s essentially like an orbit drawn for you on the sky,” says Vasily Belokurov, an astronomer at the University of Cambridge.
For the past quarter-century, astronomers have tried to use maps of Sagittarius to calculate the shape of the Milky Way’s dark matter halo. But identifying the stream from our vantage in the Milky Way’s disk is challenging, and astronomers have come up with halo shapes as varied as eggs and rugby footballs.
Then along came the European Space Agency’s Gaia satellite. Two years ago, the probe began to release its ultraprecise maps of the stars in the Milky Way—and stars in the surrounding streams. With the data, Belokurov and his colleagues could tell that the Sagittarius stream was being yanked indirectly by another gravitational player: that of the galaxy’s largest companion, the Large Magellanic Cloud (LMC), which weighs between one-fifth and one-third as much as the Milky Way itself.
Rewinding the clock, the researchers modeled the pas de trois over 3 billion years—and found that both the LMC and Sagittarius swooped close to the Milky Way, as recently as 50 million years ago. The LMC’s significant heft pulled our Galaxy, which then induced a force affecting Sagittarius. That helps explain a peculiar sideways tug on the Sagittarius stream, say Belokurov and his colleagues, who report the results in a paper posted to the preprint server arXiv. Solving this puzzle made it easier to use the Sagittarius stream as a scale and to infer the shape of the galaxy’s dark matter halo. “It’s the lock you need before you can unlock the main lock,” Belokurov says.
The team’s results suggest the distribution of dark matter around the Milky Way is complex. Closer to the disk of our Galaxy, where the dark matter is expected to be most dense, the halo takes the shape of a squashed sphere—a bit like a pumpkin, with the pumpkin’s top pointing out of the galactic plane. But farther out, about 65,000 light-years from the galactic center, the shape of the halo changes: The pumpkin tips over on its side, so that its stem is aligned with the disk of the galaxy.
The twists and turns of this convoluted shape could provide hints as to how the Milky Way’s halo is connected to the local network of dark matter filaments, called the cosmic web, that strings together neighboring large galaxies, Belokurov says.
Kathryn Johnston, an astronomer at Columbia University who was not involved in the work, agrees. “We’ve never been able to see anything beyond the simplest shape of the dark matter halo,” she says. “This is a hint of large-scale global deformation, and that’s very exciting.”
Gaining even this limited view of the Milky Way’s dark matter halo is important, Belokurov says, because it’s the closest halo we have access to: It could help researchers understand how light or heavy dark matter particles might be, and improve models that trace the evolution of the cosmic web from the big bang to today.