Milky Way's shredded companion provides clues about dark matter

Milky Way’s shredded companion provides clues about dark matter

The Milky Way hasn’t been kind to the Sagittarius dwarf galaxy. The gravity of the Milky Way has shredded and stretched the clump of stars 70,000 light-years away into a filamentous stream. Scientists have now carefully mapped Sagittarius, and they’ve used that map to offer a long-awaited picture of the mysterious dark matter halo in which our Galaxy resides.

Sagittarius, discovered in 1994, is one of the Milky Way’s nearest partners. Gravitational forces have ripped it apart throughout history, scattering stars into a stream that now entirely circles the Milky Way. Sagittarius is thus a sensitive scale for measuring the distribution of mass in our Galaxy, which includes not only the visible disk of stars, but also an invisible halo of dark matter, which is thought to account for up to 90% of total mass.

In principle, astronomers might track the orbits of nearby star clusters and galaxies and use physics laws to calculate how much matter is pulling on them. However, their movement across the sky is too slow to assist within human lifetimes. The Sagittarius stream, on the other hand, embodies such motions already. “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, scientists have attempted to calculate the shape of the Milky Way’s dark matter halo using Sagittarius maps. However, identifying the stream from our vantage in the Milky Way’s disk is difficult, and astronomers have constructed halo shapes as varied as eggs and rugby footballs.

Then came the Gaia satellite from the European Space Agency. Two years ago, the probe began to release its ultraprecise maps of the stars in the Milky Way—and stars in the surrounding streams. Belokurov and his colleagues were able to figure out from the data that the Sagittarius stream was being dragged indirectly by another gravitational player: 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, discovering that the LMC and Sagittarius were both 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. According to Belokurov and his colleagues, this helps explain a peculiar sideways tug on the Sagittarius stream, which they report in a paper submitted to the preprint server arXiv. After solving this puzzle, it was easy to use the Sagittarius stream as a scale and estimate the form of the galaxy’s dark matter halo. “It’s the lock you need before you can unlock the main lock,” Belokurov says.

According to the team’s results, the distribution of dark matter around the Milky Way is complex. Closer to our Galaxy’s disk, where dark matter is believed to be most dense, the halo takes the appearance of a squashed sphere, like a pumpkin with its top pointing out of the galactic plane. However, approximately 65,000 light-years from the galactic center, the halo’s shape changes: the pumpkin tips over on its side, with its stem aligned with the galaxy’s disk.

According to Belokurov, the twists and turns of this convoluted form could provide hints as to how the Milky Way’s halo is connected to the local network of dark matter filaments known as the cosmic web, which connects neighboring massive galaxies.

Kathryn Johnston, a Columbia University astronomer who was not involved in the research, 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.”

Even gaining this limited picture of the Milky Way’s dark matter halo is important, according to Belokurov, because it is the nearest halo to us: It could help researchers understand whether dark matter particles are light or heavy, and it could improve models that trace the evolution of the cosmic web from the big bang to today.

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