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The Milky Way Nearly Crashed Into Another Galaxy

Sep 21, 2018 10:16 PM EDT
Milky Way
Artist’s impression of disturbances in the velocities of stars in the Milky Way galaxy that was revealed by ESA’s star mapping mission, Gaia. The reason, the new study reveals, is a near collision with another galaxy millions of years ago.
(Photo : ESA (CC BY-SA 3.0 IGO))

In the 13 billion years of the Milky Way's existence, it has been through rough times, including a near collision with repercussions still echoing today.

Hundreds of millions of years ago, the Milky Way nearly crashed against another smaller galaxy. It was only a close call, but it resulted in ripples in the stars located in the Milky Way disk.

Odd Shapes Of Stellar Movement In The Data

In a paper published in the journal Nature, researchers analyzed data from the European Space Agency's Gaia mission and discovered strange movement patterns of the stars in the Milky Way.

According to ESA, Gaia allowed the scientists to observe stellar motion by plotting stars' position against their velocity, which is called phase space.

The team was surprised at the strange and interesting shapes that emerged from the study, the likes of which have never been seen before. Specifically, 6 million stars in the galaxy's disk were found to move in a pattern that looked like snail shells.

Near Collision Causes Star Ripples

When a pebble is thrown in a pond, it causes ripples in the water. After a while, though, the water calms and settles once again.

Stars display different behavior, ESA points out, with the stellar bodies retaining a memory of the disturbance. While one can't see the effect on the stars' position, it is noticeable in their unexpected velocities.

"These substructures allow us to conclude that the disk of our galaxy suffered an important gravitational disturbance about 300 and 900 million years ago," Teresa Antoja, study author and researcher from University of Barcelona in Spain, says in a statement.

Enter, the Sagittarius dwarf galaxy, a relatively tiny galaxy that is home to only a few tens of millions of stars. It is currently being cannibalized by the Milky Way, but it's not the first time the two neighboring galaxies have met.

Previous studies of the Sagittarius dwarf galaxy have revealed that its last encounter with the Milky Way didn't see impact. Instead, the small galaxy simply brushed past sometime between 200 and 1,000 million years ago.

The timeline matched the team's estimated beginning of the stars' shell-like pattern of movement, suggesting that a near collision with the Sagittarius dwarf galaxy in the distant past is still affecting the stellar behavior in the Milky Way today.

While the findings get scientists one step closer to uncovering the mysteries of the Milky Way, the authors note that the link between the snail shell shapes and the Sagittarius dwarf galaxy was achieved using simple computer models and analysis. Further studies are already being planned.

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