They found around ten billion years ago, five billion years before the Earth's formation, the Milky Way crashed into a smaller galaxy dislodging some of its stars. Although our galaxy survived, it has never been the same.
As detailed in a paper published by the journal Nature, the chemical signature of many newfound halo stars was clearly different that "native" Milky Way stars.
That's the conclusion of new research based on 2 billion measurements of how stars within the Milky Way are moving.
Over time the dwarf galaxy faded away, but the scars from its collision never really disappeared-not that they have been easy to find.
Simulations suggest that in another two billion or so years, the Milky Way will swallow the Large Magellanic Cloud, forever erasing it from the southern sky.
"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. Helmi has been involved in the development of the Gaia mission for some twenty years and was part of the data validation team on the second data release.
If those stars had been born in this galaxy, they would be marching around the core in the same direction as the rest of us. The chemical signature of many halo stars was clearly different from the "native" Milky Way stars.
"When we looked at their chemical composition, we saw that the stars defined a separate sequence in "chemical space", Helmi says.
Large galaxies get that way by absorbing lesser ones.
Columbia University astronomer Kathryn Johnson told Space.com: "The Milky Way is a cannibal". And it is one that relies on a single-yet massive-merger. "And they are a fairly homogenous group, which indicates they share a common origin". "That's when it became apparent that this was weird", she says. The big lump is called the galactic bulge, whereas the bar is called the galactic disk.
We are so deeply embedded in this collection that its stars surround us nearly completely, and so can be seen across most of the sky. Exactly which element is formed depends on the star. In every galaxy the abundance of elements heavier than hydrogen and helium gradually increases over time due to cycles of stellar death and rebirth.
Astronomers have long thought that the Milky Way had several small collisions that fed stars into the halo, but that they didn't occur recently; in fact, not for over 10 billion years (the Universe is 13.8 billion years old).
The galaxy that researchers believe collided into ours has been named Gaia-Enceladus, after the Greek goddess and her offspring. And two, are existing galaxies constructed from a few large collisions or a series of smaller ones? "It wasn't known whether the Milky Way had experienced any mergers", Helmi says. The sheer confirmation alone has sent many astronomers into a state of euphoria.
In other words, the collection is what they expected from stars that were once part of another galaxy and have been consumed by the Milky Way. "At almost three million light-years away, we're never going to Andromeda to populate it or study it in detail", says Kim Venn, an astronomer at the University of Victoria in British Columbia who was not involved in the study.
"The structure of the Milky Way's disk and stellar halo is going to change again", Besla says. Collision with object of this size had to warm up the young galactic disk, forming a thicker modern disk.
And the answer will only further shed light on these galactic wrecks.
This evidence of that ancient collision has helped solved a long-standing mystery: that of the Milky Way's thick disc.
The researchers hope that by understanding the collision in the Milky Way, they can better understand the process in other galaxies as well.