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 oneswithin the Milky Way.

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

The researchersfound 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 signthey may have originated elsewhere.

Thisclip 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 theApache 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 oneof the clues was found inthe evidence of 13 globular clustersin 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 differentgalaxy that's basically contributed all of the stars to our own galaxy,I think that was a big surprise," said lead authorAminaHelmi, an astronomer with theKapteynAstronomical Institute at the University ofGroningen in the Netherlands.

Another piece of evidencewas 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 discoveredin 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 producedbrilliant stellar explosions supernovas and the rapid birth of stars.

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

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 Wayand the neighbouring Andromeda Galaxy will have a similar collision.

The researchers namedthegalaxy that merged with oursGaia-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 illustratesthe 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,"Helmisaid. "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 thatgalactic 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."