Astronomers say they've found evidence of a series of early galactic collisions that shaped the Milky Way.

The Milky Way doesn't exist in a cosmic vacuum. Throughout its history, its interacted with other galaxies. Researchers also believe the Milky Way has been shaped by early impacts. And now, scientists have observational evidence of those impacts.

In analyzing the distribution of 3.6 million stars in the galaxy's stellar disk, researchers identified asymmetric ripples, evidence of the Milky Way's collision with the massive Sagittarius dwarf galaxy some 850 million years ago.

"These impacts are thought to have been the 'architects' of the Milky Way's central bar and spiral arms," Susan Gardner, a professor of physics and astronomy at the University of Kentucky, said in a news release. "Just as the ripples on the surface of a smooth lake suggest the passing of a distant speed boat, we search for departures from the symmetries we would expect in the distributions of the stars to find evidence of ancient impacts. We have found extensive evidence for the breaking of all these symmetries and thus build the case for the role of ancient impacts in forming the structure of our Milky Way."

The latest findings -- detailed this week in the Astrophysical Journal -- confirms earlier evidence of north-south asymmetry in the distribution of stars in the galaxy's stellar disk.

Astronomers used data collected by the Sloan Digital Sky Survey to measure the vertical distribution of stars within in relatively small regions of the Milky Way's stellar disk.

"Having access to millions of stars from the SDSS allowed us to study galactic structure in an entirely new way by breaking the sky up into smaller regions without loss of statistics," said Deborah Ferguson, lead author of the new study and a 2016 graduate of Kentucky. "It has been incredible watching this project evolve and the results emerge as we plotted the stellar densities and saw intriguing patterns across the footprint. As more studies are being done in this field, I am excited to see what we can learn about the structure of our galaxy and the forces that helped to shape it."

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