A COSMIC poltergeist has rearranged stars in the Milky Way, making odd stellar layers near the top and bottom of the galaxy’s spiral disc. The main suspect is a galaxy that was ripped apart by our own, whose remains lie eerily close to the most disturbed zones.
Over time, gravitational tugs from the Milky Way’s bulk of regular and dark matter should have smoothed out the vertical distribution of stars. But last year astronomers looking at data from the Sloan Digital Sky Survey noticed that there seemed to be more stars clustered below than above our position in the galactic disc. A little further away, there are more stars above than below.
“It sounds like an oscillation, and that’s exactly what the data look like,” says Susan Gardner of the University of Kentucky in Lexington. Gardner and Brian Yanny of the Fermi National Accelerator Laboratory in Batavia, Illinois, have now looked at an even larger sample of stars. They found that there is a series of layers that trace the high and low points in an oscillation, and that stars closer to the galaxy’s edge are in larger groupings than those near the centre (arxiv.org/abs/1309.2300).
The remains of a dwarf galaxy called Sagittarius lie near the biggest groups, and other work suggests that this object flew through the Milky Way’s disc as it was being consumed. Future observations will help reveal whether the dwarf was massive enough to be the culprit.