Gaia-Enceladus Dwarf Galaxy Merges with the Milky Way

Approximately 10 billion years ago, a dwarf galaxy roughly the size of the Small Magellanic Cloud collided with and was absorbed by the young Milky Way. The merger deposited an estimated 50 billion solar masses of stars, gas, and dark matter, along with at least eight globular clusters. Evidence for this event was identified in 2018 through Gaia spacecraft proper motion data, which revealed a distinct population of stars on highly radial orbits with chemical abundances unlike native Milky Way stars. The collision reshaped the Milky Way's inner halo and triggered waves of new star formation across the galactic disk.