So, you think you hear ringing in your ears – it’s the Milky Way?

An international team of astronomers have discovered evidence that our Milky Way had an encounter with a small galaxy or massive dark matter structure perhaps as recently as 100 million years ago, and as a result of that encounter it is still ringing like a bell.

The discovery is based on observations of 300,000 nearby Milky Way stars by the Sloan Digital Sky Survey. Stars in the disk of the Milky Way move up and down at a speed of about 20-30 kilometers per second while orbiting the center of the galaxy at a brisk 220 kilometers per second. The positions and motions of these nearby stars weren’t quite as regular as previously thought, according to the study results.

“We clearly observe unexpected differences in the Milky Way’s stellar distribution above and below the Galaxy’s mid-plane having the appearance of a vertical wave – something that nobody has seen before,” says Queen’s University physicist Larry Widrow, lead researcher on the project.

The researchers have not been able to identify the celestial object that passed through the Milky Way. It could have been one of the small satellite galaxies that move around the center of our galaxy, or an invisible structure such as a dark matter halo. It might not have been a single isolated event in the past, and it may even be ongoing…

Computer simulations indicate that over the next 100 million years or so, our galaxy will “stop ringing”. The north-south asymmetry will disappear and the vertical motions of stars in the solar neighborhood will revert back to their equilibrium orbits.

Unless we get hit again.

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