Gaia reveals major galaxy 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, as reported 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.
The salient features of this extraordinary event are outlined in several new papers, some of which were led by a Institute of Astronomy graduate student, GyuChul Myeong (supervised by Vasily Belokurov and Wyn Evans). He and colleagues 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. Thanks to Gaia, astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy.
“The paths of the stars from the galactic merger earned the moniker ‘Gaia Sausage’," explained Wyn Evans of the Institute of Astronomy. "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.”
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 (including gas, stars and dark matter) would have been 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’.
The head-on collision of the Sausage galaxy was a defining event in the early history of the Milky Way, creating both the thick disk and the inner stellar halo. Even though the merger took place in the very remote past, the stars from the Sausage galaxy can be picked out today. Evidence of this event lies all around us in the kinematics and chemistry of 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.
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