OUR galaxy seems to be more milkshake than Milky Way, opening up a fresh cosmic mystery. The first three-dimensional map of stellar motion in our galactic neighbourhood shows that stars are all shaken up, and hints that the galaxy is undulating like a flag in a breeze.
Mary Williams at the Leibniz Institute for Astrophysics Potsdam in Germany and her colleagues combined recently released data about the horizontal speeds of thousands of stars with other readings of how the stars are moving up and down. They found that stars closer to the centre of the galaxy are spreading outwards above and below the galactic 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, a section of the galactic disc would resemble a rippling flag (Monthly Notices of the Royal Astronomical Society, doi.org/ph7).
The motion could be produced as two satellite galaxies, called the Magellanic Clouds, spiral around the Milky Way and distort its disc. Or dark matter lumps from a halo surrounding our galaxy could be passing through and kicking up a fuss. Such scenarios can be tested by refining galactic models with possible culprits in mind and comparing them with observations, says Ralph Sutherland at the Australian National University in Canberra.
This article appeared in print under the headline "Milky Way shake-up set our galaxy fluttering like a flag"
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