Scientists discovered the major collision that changed the Milky Way

While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all.

Artist's impression of a collision between the Milky Way and a massive dwarf Credit: V. Belokurov (Cambridge, UK) based on an image by ESO/Juan Carlos Muñoz
Artist's impression of a collision between the Milky Way and a massive dwarf Credit: V. Belokurov (Cambridge, UK) based on an image by ESO/Juan Carlos Muñoz

An international team of astronomers including the University of Cambridge has discovered an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed ‘the Sausage galaxy’.

The astronomical crash was a characterizing occasion in the early history of the Milky Way and reshaped the structure of our world, designing both the universe‘s internal bulge and its external radiance, the stargazers report in a progression of new papers.

According to astronomers, around eight to 10 billion years prior, an obscure dwarf galaxy crushed into our own particular Milky Way. The dwarf did not survive the effect. It rapidly went into disrepair, and the destruction is presently surrounding us.

Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City said, “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 center 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.”

Scientists used data from the European Space Agency’s Gaia satellite. This spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. In addition, using Gaia, astronomers now know the positions and trajectories of our celestial neighbors with unprecedented accuracy.

Wyn Evans of Cambridge’s Institute of Astronomy said, “The paths of the stars from the galactic merger earned the moniker ‘Gaia Sausage’. We plotted the velocities of the stars, and the sausage shape just jumped out at us. As the smaller galaxy broke up, its stars were thrown out on very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way.”

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 direction implied that the Milky Way’s disk was most likely puffed or even cracked after the effect, and the Milky Way needed to re-grow another disk. In the meantime, the Sausage debris was scattered all around the inward parts of the Milky Way, making the ‘bulge’ at the galaxy’s middle and the encompassing ‘stellar halo’.

In addition, scientists 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.

Numerical simulations of the galactic smash-up can reproduce these features,” said Denis Erkal of the University of Surrey. In simulations ran by Erkal and colleagues, stars from the Sausage galaxy enter stretched out orbits. The orbits are further elongated by the growing Milky Way disk, which swells and becomes thicker following the collision.

“Evidence of this galactic remodeling is seen in the paths of stars inherited from the dwarf galaxy,” said Alis Deason of Durham University. “The Sausage stars are all turning around at about the same distance from the center of the Galaxy. These U-turns cause the density in the Milky Way’s stellar halo to drop dramatically where the stars flip directions.” This discovery was especially pleasing for Deason, who predicted this orbital apocentric pile-up almost five years ago.

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. The memory of this event persists in the kinematics and chemistry of its stars. Thanks to the Gaia satellite, astronomers have miraculous data with which we can peer back into the very distant past and recreate the pre-history of our galactic home.