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Gaia Enceladus (aka Gaia-Sausage-Enceladus, abbreviated GSE) is a presumed past dwarf galaxy that merged with the Milky Way gigayears ago (the GSE Merger). Its past existence was deduced from Gaia data showing Milky-Way stars with particular orbit characteristics. The merger occurred 8 to 11 billion years ago, substantially larger than any subsequent merger. Billions of stars as well as several globular cluster were added.
The word sausage was inspired by a plot of the velocities of the stars (circular motion versus radial motion), in which the stars attributed to Gaia Enceladus occupy a distinct large oval (sausage-shaped) region of the plot.