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When our Milky Way merged with an ancient dwarf galaxy

7 Aug 2019, 10:36 UTC
When our Milky Way merged with an ancient dwarf galaxy
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In 2018, astronomers proposed that that, in its early history, our Milky Way galaxy collided with and devoured a dwarf galaxy, thought to have been slightly more massive than the Small Magellanic Cloud. They call this hypothetical dwarf galaxy Gaia-Enceladus. Tantalizing evidence of this collision is a cluster of blue stars present in the Milky Way’s halo, which is a nearly spherical region of thinly scattered stars, globular clusters of stars, and tenuous gas surrounding our Milky Way. While scientists believed the collision and its subsequent merger led to the formation of our galaxy’s thick disk, the precise ages of these stars were unclear, until now.
Scientists from Instituto de Astrofisica de Canarias (IAC) in Spain used data from the Gaia spacecraft and pinned our galaxy’s most ancient stars – remnants of the merger – to be 10 to 13 billion years old. This provides evidence that the Milky Way–Gaia-Enceladus collision occurred 10 billion years ago. A statement from these scientists explains:
Thirteen billion years ago, stars began to form in two different stellar systems which then merged: one was a dwarf galaxy which we call Gaia-Enceladus, and the other was the main progenitor of our galaxy, some four times ...

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