A visualisation of our hit-and-run companion galaxy (Image: Erik Tollerud)





Video: How the Milky Way got its spiral arms

A dwarf galaxy called Sagittarius can be credited with giving the Milky Way its signature spiral arms.

Sagittarius struck our galaxy some 1.9 billion years ago. It then looped over the galactic “north pole” and struck again about 900 million years ago. It is heading back right now, on course for a third clash in 10 million years or so.


These impacts must have had a considerable influence on the Milky Way but the effects have been hard to calculate because of uncertainties over the amount of dark matter in Sagittarius.

Now Chris Purcell at the University of Pittsburgh, Pennsylvania, and colleagues used new mass estimates for Sagittarius to create the most accurate simulation yet of the effects of the impacts (see visualisation, above).

Galactic evolution

The simulation starts with the Milky Way as a flat disc with a central bar of stars and gas. After the first impact, instead of orbiting in circles around the central bar, some stars start orbiting in a variety of ellipses. These combine to form dense clumps of stars and gas in a spiral pattern.

After the second impact, the spirals become remarkably similar to the ones seen in the Milky Way. At the same time, the central bar is preserved, just as in the actual Milky Way.

The impacts also generate ring-like structures around the Milky Way. One of these, says Purcell’s team, is very similar to a feature of the night sky known as the Monoceros ring. The simulation suggests that the ring connects to the spiral arms – beyond the view of older telescopes. Purcell predicts that the next generation of galactic maps will show the connection.

“The work is impressive because it reproduces many of the features we can actually see,” says Steve Majewski, an astronomer at the University of Virginia in Charlottesville. “That’s never been done before.”

The work has broader implications for our understanding of galactic evolution because impacts between galaxies and their companions are thought to be widespread in the cosmos. “Many of the spiral galaxies we can see were probably formed in this way,” says Purcell.

Journal reference: Nature, DOI: 10.1038/nature10417