MEGA MERGER
Astronomers discover that our Milky Way was formed after COLLIDING with another galaxy 10 billion years ago
The researchers made their discovery by tracking the motions of stars and examining their chemical composition
ASTRONOMERS have discovered that the Milky Way was formed after colliding with another galaxy, some 10 billion years ago.
The team of researchers came to their conclusions by looking at the movement and composition of our galaxy's stars, finding that many of these shared a chemical makeup different from those of its 'native' stars.
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Based at the University of Groningen in Holland, the astronomers set out to discover how the Milky Way had formed.
Scientists were already confident that our galaxy was the result of mergers between separate galaxies, but the Dutch team wanted to determine whether it was formed by many collisions or just one big one.
They did this by looking at "chemo-dynamical" composition of the galaxy, meaning that they tracked the trajectory of its stars and examined visual signs of its chemical makeup.
By doing this, they proved that the 'halo' of stars surrounding the Milky Way's core came from a younger galaxy, which is known as Gaia-Enceladus (named after the Giant Enceladus of Greek mythology).
Simulation of a merger between galaxies resembling the Milky Way and Gaia-Enceladus
"We expected stars from fused satellites in the halo. What we didn't expect to find was that most halo stars actually have a shared origin in one very large merger," said Prof. Amina Helmi, the lead author of the research.
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Her team also discovered that many of the Milky Way's outer stars were formed more recently than the stars that make up the galaxy's core.
Prof. Helmi explains, "The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region. This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up."
Impressively, the team arrived at their conclusions by examining data on 1.7 billion stars, which was obtained by the European Space Agency's Gaia satellite.
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Yet this data reminded Prof. Helmi of a galaxy-merger simulation produced ten years ago by a PhD student, which was consistent with the information her team studied.
"It was amazing to look at the new Gaia data and realize that I had seen it before," she said.
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