A galactic collision influenced history of the Milky Way

According to a team of scientists, the Milky Way evolved about 10 billion years ago when it collided head-on.
By Karen Saltos | Jul 10, 2018
As the Milky Way evolved about 10 billion years ago, it collided head-on with another smaller galaxy. That cataclysm changed its structure forever.

Two new studies describe the overlooked evidence for this event. "There's debris everywhere," said Vasily Belokurov, an astronomer at the University of Cambridge and a leader of one of the two teams.

To uncover evidence of the collision, astronomers had tosift through multitudinous sources of surviving information.They can use it to compose a story consistent with the available evidence.

Both research teams used information from the European Space Agency's Gaia space telescope. It has been gathering biographies of millions of stars for several years.

The experiments involved creating high-resolution and multidimensional maps of the Milky Way and using them to find anomalous populations of old stars that seem to retain a memory of the collision. The team found a large number of stars that do not move coordinated with the galaxy's rotation. Instead, they move in radial orbits, streaming toward or away from the center of the galaxy.

Belokurov's group also modeled different collision scenarios, as well as a possible quieter history without significant collisions. An impact of a small "dwarf" galaxy could have deposited a cloud of stars like the ones seen today, they discovered.

The collision theory could help resolve a continuing question about the Milky Way's structure. Its spiral disk of stars is inclusive of two parts: a thin dense region encompassed by a thick diffuse region.

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