Galactic map reveals a wave of motion within the Milky Way

New Scientist : A new three-dimensional map of speeds and locations of stars in the Milky Way suggests that there is a galaxy-traversing density wave that doesn’t align with the normal rotational motion. The Radial Velocity Experiment (RAVE) measured nearly half a million stars within 6500 light-years of Earth. Mary Williams of the Leibniz Institute for Astrophysics Potsdam in Germany and her colleagues examined RAVE’s data and found that stars closer to the galactic center are moving outward above and below the galactic plane. Farther out, stars are moving in toward the center. The undulation could be the aftereffect of a previous collision with a smaller galaxy, or it could be a continual effect caused by the Magellanic Clouds . A third possibility is that the motion could be caused by dark matter passing through the galaxy. Further evidence of the wave may come from the European Space Agency’s Gaia spacecraft, which will provide a larger view of the universe.