Collisions of the dwarf galaxy Sagittarius with the Milky Way gave our Galaxy its iconic spiral arms, say astronomers reporting results of new supercomputer simulations in the journal Nature.

The collision of Sagittarius with the Milky Way is not a new concept; previous observations tracing the orbital motion of stars revealed that it has collided with the Milky Way twice before, but studies usually focus on the effects imparted on the smaller dwarf galaxy. The new simulations, conducted by then PhD student Chris Purcell of the University of California in Irvine, who is now based at the University of Pittsburgh, focuses instead on the effects inflicted upon the Milky Way.

The dwarf galaxy was initially loaded with dark matter, the hypothesized, invisible matter inferred from its gravitational influence on stars and galaxies. As it slammed into the Milky Way, up to 90 percent of the dark matter was stripped away from the dwarf, setting up instabilities in the flat Milky Way disc. Stars streamed into long loops around the Galaxy, its rotation gradually pulling them into the familiar spiral arm shapes.

"Dark matter was absolutely vital to the formation of the spiral arms," Purcell tells Astronomy Now. "When Sagittarius first fell in toward the Milky Way, it was likely about 100 times more massive in dark matter than stars. Had there been no dark matter in the dwarf galaxy, the impact on the Milky Way disc would have been negligible."

Purcell says that the spiral arms began to form immediately after the initial collision, although the complete emergence of the multi-armed spiral structure did not become evident until about a billion years after the first impact. "The second impact is much less dramatic since Sagittarius has lost most of its dark matter mass by then, although you do see a secondary (and much weaker) mode of spirality emerge immediately following the second impact," he says.

Sagittarius has passed through our Milky Way twice during the last two billion years – once 1.9 billion years ago and again 0.9 billion years ago – and is taking aim for a third shot at the southern edge of our Milky Way disc in another ten million years. But since Sagittarius has been stripped of its dark matter mass, and most of its stellar mass too, it will unlikely influence the Milky Way further.

Purcell suspects that a large fraction of the spiral galaxies in the local Universe have been significantly perturbed into spirality by small companion satellites in the process of merging with the main system.  "It could be that future efforts will allow us to make more concrete statements about whether a particular spiral galaxy is likely to have been impacted, or whether it is likely to have evolved that way in isolation," he says. "Hopefully learning more about the Milky Way and the Sagittarius influence will help us figure what to probe next, in the context of nearby large disc galaxies."