A Dwarf Galaxy Crashed Into The Milky Way 10 Billion Years Ago

A still from a simulation of the collision. H.H. Koppelman, A. Villalobos and A. Helmi

A group of scientists has suggested that the Milky Way was hit by a dwarf galaxy about 10 billion years ago, the last big impact to rock our galaxy.

Using data from the European Space Agency’s (ESA) Gaia telescope, the team led by Amina Helmi from the University of Groningen studied the motion of stars in our galaxy. They discovered some may have come from another galaxy, dubbed Gaia-Enceladus (GE). The findings are published in Nature.

“The inner halo of our galaxy is formed by stars that were born in a different galaxy,” Helmi told IFLScience. “It’s really thanks to Gaia we can do this [study].”

Back then, the team think that GE was about one-quarter the size of the Milky Way – no impact since is thought to have been as big – and similar in size to the Small Magellanic Cloud, one of our neighboring satellites. The Milky Way has grown, however, with the GE about a tenth the size of our modern galaxy.

The size difference at the time though would have dramatically altered the appearance of our galaxy, described as a “major event” by Helmi. And it helps us paint a picture of how our Milky Way evolved.

The merger not only populated the inner region of our galaxy with stars, but it also caused part of the Milky Way’s disk to become thicker, altering its appearance. Our galaxy is composed of two disks, a smaller thin disk and a larger thick disk, but finding the origins of the latter has been difficult.

To make their findings, the team looked at the motion and position of 7 million stars, finding that about 30,000 were moving in opposite directions compared to the rest within the thick disk. These stars are near our Sun right now, meaning many can be seen in the night sky.

Looking at the composition of the stars using a survey called APOGEE, the team were also able to work out that they had a different chemical composition to those in our galaxy, suggesting they originated elsewhere. Based on these compositions, the researchers were even able to work out when the collision occurred.

Our galaxy is thought to have been shaped by several galaxies, but based on the shape of the disks we don't think it's been so dramatically changed since GE. "I suspect [the merger] is one of many,” said Helmi, “but it’s the largest recent one.”

The study also highlights just how successful ESA's GAIA telescope has been. Already it is giving us a brand new look at 1 billion stars in our galaxy, conducting the most detailed analysis ever, and it's helping us piece together the history of the galaxy we call home.

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