Our galaxy seems to be more milkshake than Milky Way, opening up a fresh cosmic mystery. A three-dimensional map of the speeds and distances of thousands of stars suggests that they are all shaken up and that the galaxy is undulating up and down, but no one knows why. The wavy motion could be driven by previous collisions with smaller galaxies or by clumps of dark matter. Figuring out the cause will tell us more about the Milky Way's history and will inform future surveys of our galactic home.
The Milky Way is a large, spiral galaxy that roughly resembles a flattened disc of stars and gas. We know that the disc is rotating horizontally around a central region that contains a supermassive black hole. But no one was sure whether the disc also has vertical motion.
"If there are no external impacts on the disc, what you'd expect is a steady state: things would be going around and around – but there shouldn't be up-and-down motion. That's hard to induce," says Kenneth Freeman at the Australian National University in Canberra.
Sloshing stars
Previous work examined the vertical distribution of stars in our corner of the galaxy and saw compressed layers that hinted at a wave moving through the disc. Now Mary Williams at the Leibniz Institute for Astrophysics Potsdam in Germany and her colleagues have examined recently released data from the Radial Velocity Experiment (RAVE) survey to create the first 3D map of stellar motion in our galactic neighbourhood.
RAVE covers almost half a million stars spanning 6500 light years in all directions. The team focused on a category of stars called red clump giants, which have about the same brightness and so are easier to compare when calculating their relative speed and distance from us. They then combined the RAVE data on horizontal motion with other readings of how the stars move up and down.
What they found is that stars closer to the centre of the galaxy are spreading outwards above and below the plane while stars further from the centre are squashing inwards. The motions of individual stars within these zones are chaotic, with some sloshing around in odd directions. But if we could see the overall pattern from the outside, our section of the galactic disc would resemble a flag rippling in the breeze.
Diving dark matter
It is possible the wave is a lingering effect from a galaxy that smashed into ours in the past. Or it could be actively produced as two satellite galaxies, called the Magellanic Clouds, spiral around the Milky Way and distort its disc.
Freeman thinks the answer may be something more exotic. A halo of dark matter
A next step will be figuring out whether the whole galaxy is waving or just our neighbourhood. That's something we might glean from the European Space Agency's Gaia spacecraft, due to launch this year. We can also look for evidence of a past collision in the right place to have triggered the wave.
Then the various scenarios can be tested by refining our galactic simulations and comparing them with observations of the wave and any possible culprits, says Ralph Sutherland, also at the Australian National University in Canberra. "They've really thrown down the gauntlet to the theorists and modellers," he says.
Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/stt1522
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