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

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 the galaxy’s inner bulge and its outer halo.

Ultraviolet image of the globular cluster NGC 1851 in the southern constellation Columba. NGC 1851 is a Sausage globular cluster. Image credit: NASA/JPL-Caltech/SSC

The astronomers propose that around eight 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.

The collision ripped the dwarf to shreds, leaving its stars moving on very radial orbits, like needles. These stars’ paths take them very close to the centre of our galaxy. This is a tell-tale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.

There are ongoing mergers taking place right now, such as between the puny Sagittarius dwarf galaxy and the Milky Way. However, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter was more than 10 billion times the mass of our sun.

When it crashed into the young Milky Way, it caused a lot of mayhem. The Sausage’s piercing trajectory meant that the Milky Way’s disk was probably puffed up or even fractured following the impact, and the Milky Way had to re-grow a new disk.

At the same time, the Sausage debris was scattered all around the inner parts of the Milky Way, creating the ‘bulge’ at the galaxy’s centre and the surrounding ‘stellar halo’.

Evidence of this galactic remodelling is seen in the paths of stars inherited from the dwarf galaxy. The Sausage stars are all turning around at about the same distance from the centre of the Galaxy. These U-turns cause the density in the Milky Way’s stellar halo to drop dramatically where the stars flip directions.

The new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies do not normally have globular clusters of their own, so the Sausage galaxy was big enough to host its own entourage of clusters.

The head-on collision of the Sausage galaxy was a defining event in the early history of the Milky Way. It created the thick disk and the inner stellar halo. Even though the merger took place at a very remote epoch, the stars in the Sausage galaxy can be picked out today.

Memory of this event persists in the kinematics and chemistry of its stars. (University of Cambridge)

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