Milky Way was part of cosmic collision 10 billion years ago

Astronomers using the European Space Agency's Gaia space telescope have discovered that our galaxy was involved in a cosmic merger 10 billion years ago.

The Milky Way is believed to have formed more than 13 billion years ago. But shortly thereafter — shortly in cosmic terms, at least — another galaxy slammed into it, dispersing its stars while also creating new ones within the Milky Way.

Spiral galaxies like our own are made up of several parts: the central bulge, spiral arms, the disk and a surrounding halo.

The researchers found evidence of the merger by studying the movement of seven million stars in the the Milky Way's inner halo, a region around the galaxy's thick disk of stars. They discovered that about 30,000 of these stars were moving in the opposite direction of the other stars in the galaxy — a clear sign they may have originated elsewhere.

This clip shows a simulation of the merger of a Milky Way-like galaxy (with its stars in blue) and a smaller galaxy (with its stars in red). Initially, the two galaxies are clearly separated, but gravity pulls them together and they merge.

Before this discovery, the team had run simulations of galactic mergers. What they observed with the Gaia data — and with data from the Apache Point Observatory's Galactic Evolution Experiment (APOGEE) in Chile — matched these simulations, leading them to conclude another galaxy had merged with the Milky Way.

The researchers say one of the clues was found in the evidence of 13 globular clusters in the same region of the galaxy — thousands to millions of stars all bound tightly together by gravity — that move in the same manner as the 30,000 stars they observed.  

"The discovery that ... the inner halo of the Milky Way turns out to be a different galaxy that's basically contributed all of the stars to our own galaxy, I think that was a big surprise," said lead author Amina Helmi, an astronomer with the Kapteyn Astronomical Institute at the University of Groningen in the Netherlands.

Another piece of evidence was the composition of the stars themselves. Stars from different galaxies have their own kind of fingerprint. And that was the case with the 30,000 odd-moving stars they discovered in the Milky Way's halo.

"It's very cool that stars that formed in another galaxy could be lurking right next door to us," said Kim Venn, a professor at the University of Victoria's department of physics and astronomy, who was not involved in the study.

Stellar explosions

The merger would have produced brilliant stellar explosions — supernovas — and the rapid birth of stars.

"If you were there … you would see bright, blue young stars. Kind of like fireworks," Helmi said.

Three billion years after the Big Bang, galaxy mergers were starting to slow down, but they were much more common than they are today. In about four billion years, the Milky Way and the neighbouring Andromeda Galaxy will have a similar collision.

The researchers named the galaxy that merged with ours Gaia-Enceladus, after both the telescope and the mythical Greek figure who was the son of Gaia, the mother of all life.

Gaia-Enceladus is believed to have been roughly the size of one of the Magellanic Clouds, two galaxies that are satellites of our own and roughly 10 times smaller.

But 10 billion years ago, the Milky Way was itself much smaller, which illustrates the explosive, star-creating power of the merger.

The researchers hope that by understanding the collision in the Milky Way, they can better understand the process in other galaxies as well.

​"The other thing we'd like to do is go beyond this 10 billion years to earlier and earlier and see if we can find evidence of mergers that took place early on and what those mergers looked like," Helmi said. "By studying these stars that were present in these galaxies, you get a way of understanding the properties of galaxies."

The research, published today in the journal Nature, is far from finished. With the latest Gaia data release, there's much more to do.

"What I really like is that Gaia data was combined with the  … APOGEE data," Venn said. "It took both surveys working together, and both of these projects will continue, so we could find more stars from the merged galaxy and probably start to reconstruct its whole history."

And that galactic sleuthing is exactly what's so enticing to Helmi.

"The Milky Way is our home, and people like to know their origins; they like to know their own history. And to me, that's what's fascinating."