The Gaia Sausage: The major collision that changed the Milky Way galaxy

An international team of astronomers has discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed the "Sausage" galaxy. The cosmic crash was a defining event in the early history of the Milky Way and reshaped the structure of our galaxy, fashioning both its inner bulge and its outer halo, the astronomers report in a series of new papers.

The astronomers propose that around 8 billion to 10 billion years ago, an unknown dwarf galaxy smashed into our own Milky Way. The dwarf did not survive the impact: It quickly fell apart, and the wreckage is now all around us.

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Credit: V. Belokurov (Cambridge, UK); Based on image by ESO/Juan Carlos Muñoz

An impression of the encounter between the Milky Way galaxy and the smaller Sausage galaxy about 8 billion to 10 billion years ago. The record of this ancient encounter is still preserved in the velocities and chemistry of the stars.

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Credit: V. Belokurov (Cambridge, UK) and Gaia/ESA

When looking at the distribution of star velocities in the Milky Way, the stars of the Sausage galaxy form a characteristic sausage-like shape. This unique shape is caused by the strong radial motions of the stars. As the sun lies in the centre of this enormous cloud of stars, the distribution does not include the slowed-down stars currently making a U-turn back toward the galaxy's centre.

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A numerical simulation of an accretion event similar to the Gaia Sausage.

The Milky Way is shown in grey-scale while the accreted stars are shown in red.

The stars from the accreted galaxy are ripped off and left on extremely radial orbits.

The sausage arises when these stars are viewed in velocity space since they have much larger velocities in the radial direction than in any other direction