The Milky Way did not grow into the vast galaxy we see today on its own. Our galaxy swallowed a smaller galaxy about 11.8 billion years ago, marking the earliest major merger yet identified in the Milky Way’s history, new research has found.
The galaxy that became a meal was named Low-energy-Kraken-Heracles (LKH) and contained stars with a combined mass roughly equivalent to 50 crore Suns, or about one-quarter that of the young Milky Way. The finding pushes the known merger history of our galaxy back to a remarkably early period, when the universe was less than 2.1 billion years old at the time, which is about 15% of its current age of around 13.8 billion years.
The event happened about 1.5 billion years after the Big Bang, when the Milky Way itself was still taking shape.
Rather than observing the collision directly, astronomers reconstructed it from the remains it left behind. They studied ancient globular clusters, which are tightly packed groups of very old stars, in the inner regions of the Milky Way. Using observations from the Hubble Space Telescope and the European Space Agency’s (ESA) Gaia mission, researchers examined the clusters’ ages, chemical composition and movements.
These clues revealed a distinct population of stars that could not have formed as part of the original Milky Way alone. The researchers concluded that these stars were deposited by the LKH galaxy during the merger.
A galaxy merger does not mean stars simply smash into one another as the distances between stars are enormous.
Instead, gravity pulls the galaxies apart and mixes their stars, gas and dark matter.
In the LKH event, the stellar merger was relatively gentle, but the interaction between gas clouds is thought to have triggered fresh star formation.
The smaller galaxy also left much of its material in the Milky Way’s inner region, helping shape the galaxy that developed later.
The discovery shows that the Milky Way’s growth was driven not just by stars forming within it, but also by smaller galaxies being absorbed.
LKH was not the last galaxy to become part of the Milky Way. Gaia has revealed evidence of other mergers, including Gaia-Sausage-Enceladus, which collided with the Milky Way roughly 10 billion years ago and significantly changed its structure. Source: India Today
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