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Star-Crossed: Milky Way's Spiral Shape May Result from a Smaller Galaxy's Impact

Encounters with the Sagittarius Dwarf Galaxy could have had huge effects on the structure of the Milky Way
A simulation of the Sagittarius Dwarf Galaxy falling into the Milky Way

Erik Tollerud

The lovely, familiar swirl of the Milky Way, with its symmetric spiral arms winding outward from a central bulge, may be scars from a smaller galaxy punching above its weight. A new computer re-enactment of billions of years of galactic evolution suggests that the Milky Way owes much of its current shape to interactions with a nearby dwarf galaxy.

The Sagittarius Dwarf Galaxy, first discovered in 1994, is a satellite galaxy that is slowly being torn apart and ingested into the larger Milky Way. In the process, however, Sagittarius seems to have been making its presence felt. A group of astrophysicists at the University of Pittsburgh, the University of California, Irvine, and Florida Atlantic University simulated the gravitational infall of Sagittarius over the past few billion years to uncover what effects the dwarf galaxy may have had on the Milky Way. [Read more about the structure of the Milky Way.]

The effects, as it turned out, were strong. In the simulations, described in a study published in the September 15 issue of Nature, Sagittarius stirred up enough ripples to make a smooth, circular, spinning galactic disk evolve into a spiral much like the Milky Way. (Scientific American is part of Nature Publishing Group.) The resulting galactic perturbation also resulted in the development of loose strands of stars at its periphery that resemble an outer Milky Way feature known as the Monoceros ring. [See a video of the simulation below.]

Without the influence of a Sagittarius-type satellite, the simulated galaxy remained a flat, rather uniform disk that little resembled our galaxy. "We just ran the disk in isolation, and it stays pretty much globally smooth," says lead study author Chris Purcell, a University of Pittsburgh astrophysicist. "You certainly don't see any spiral arm formation." Had it not been for Sagittarius, then, the Milky Way might never have taken its familiar, whirlpoolesque form.

The study demonstrates that as large galaxies consume their smaller neighbors in so-called minor mergers that are common throughout the universe, the bully in the galactic interaction does not escape unscathed. "We've known for awhile that minor mergers can have visible effects on their host galaxies," says David Law, an astrophysicist at the University of Toronto's Dunlap Institute for Astronomy and Astrophysics, who did not contribute to the new study. "But this is one of the first times that we've been able to make a good link between a specific minor merger and a specific effect."

The dwarf galaxy's outsize influence stems from the assumption that although Sagittarius today is a mere fraction of the Milky Way's mass, it should once have rested inside a hefty cocoon of dark matter, known as a dark matter halo, some 100 billion times the mass of the sun. (Dark matter is a mysterious, theorized substance thought to account for one quarter of the universe's mass, some five times as much as ordinary matter provides.) Sagittarius's merger with the Milky Way is not a simple collision—the dwarf galaxy has followed a looping, spiraling inward orbit for the past few billion years that has drawn it repeatedly into contact with the Milky Way. As Sagittarius approached the Milky Way, passed through its disk, and circled back again, the dark halo of Sagittarius would have slammed into the plane of the Milky Way twice, knocking the disk askew and stirring up the formation of its spiral arms.

"We have this dramatic perturbation to the entire disk—it's coming straight down onto it in the last two impacts at least," Purcell says of the circuitous path around and through the Milky Way that Sagittarius took in the simulations. "You can't really get away from causing a spiral structure if you have an impact from a galaxy that's as massive as we think Sagittarius was."

It remains to be seen whether spiral galaxies across the universe owe their distinctive shape to similar events, or whether other effects can trigger the formation of spiral arms. "My feeling is that it shouldn't be a necessary condition," Law says. "People are still trying to figure out exactly what drives the evolution of spiral structure. It doesn't seem like on the basis of simulations that you need to have a satellite galaxy impact."

Another potential case study lies just 2.5 million light-years away. Purcell says that he and his colleagues may soon shift their focus to Andromeda, the nearest spiral-galaxy analogue to the Milky Way. "We're interested in knowing how common these events are in the bigger picture," he says. Perhaps, after all, a relatively recent galactic merger is responsible for Andromeda's structure—and the structure of countless other galaxies—as well.

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