Milky Way swallows Gaia-Enceladus
A dwarf galaxy collides with the young Milky Way about 10 billion years ago. Its stars form much of the inner halo and the impact helps puff up the galaxy's thick disk.
The story
About 10 billion years ago, when the Milky Way was young, a dwarf galaxy crashed into it and was torn apart. The collision was discovered in 2018 using data from the European Space Agency's Gaia satellite, launched in 2013, which measured the positions and motions of over a billion stars. Two teams found a large group of stars in the Milky Way's inner halo moving on stretched, nearly radial orbits, unlike the rest. One team, led by Vasily Belokurov, called it the 'Gaia Sausage' because of its shape in a velocity plot. Another, led by Amina Helmi, named it Gaia-Enceladus, after the giant of Greek myth buried under Mount Etna. Today the combined name is Gaia-Sausage-Enceladus.
The dwarf galaxy was large, with stars equal to perhaps a billion Suns. As it fell in, its gravity stirred the Milky Way. The stars already in the early disk were heated and flung into puffier orbits, which is how the thick disk is thought to have got its shape, and the impact may have set off a burst of star formation. Its own stars and globular clusters were scattered, and now make up a large part of the inner halo. Their distinct chemistry lets astronomers trace them.
Why it mattered
- It showed that the Milky Way was shaped by a major merger early in its life, and not only by quiet growth.
- Astronomers now use stellar motions and chemistry to reconstruct the galaxy's past, a field called galactic archaeology.
- The merger helps explain the thick disk, which holds many of the galaxy's older stars.
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