Above the horizontal plane that slices through the center of the galaxy, stars closer to the center of the galaxy than the sun are, in general, moving away from the plane at speeds of 10 kilometers per second or less. Meanwhile, those farther from the galactic center than the sun are moving toward the plane - in some cases, as fast as 17 kilometers per second.
Altogether, the complexity of motions observed by the team is similar to that seen among molecules in a gas with a sound wave passing through it, the researchers report this month in the Monthly Notices of the Royal Astronomical Society.
As of yet, the reasons for these anomalous motions aren't clear, the researchers note. The "wave" may indeed be a ripple caused by a long-ago collision with a small companion galaxy, or it may result from perturbations in pressure triggered as the Milky Way's spiral arms push their way through space as the galaxy rotates.