These Sausage stars are what's left of the last major merger of the Milky Way.
- Wyn Evans
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,” said Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City. “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.”
The salient features of this extraordinary event are outlined in several new papers, some of which were led by Cambridge graduate student GyuChul Myeong. 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’,” said Wyn Evans of Cambridge’s 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.”
Image: 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
Reproduced courtesy of the University of Cambridge