A new study led by researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC) reveals how the discs of galaxies like the Milky Way are affected by ancient galactic collisions.
Published in the Monthly Notices of the Royal Astronomical Society , the study analyses, using simulations, how galaxy collisions can completely or partially destroy stellar discs. Together with observational data on star clusters, the authors use this study to improve predictions about the timing of the last significant galactic collision in the Milky Way.
When did the Milky Way's disc spin up?
The disc of the Milky Way is a vast, rotating, pancake-shaped system of stars, with spiral arms winding out from its centre. This disc contains the majority of the galaxy's stars, including the Sun, and rotates at over 220 kilometres per second.
For a long time, astronomers have tried to determine when this rotating disc formed. A key clue lies in the motions and ages of the stars: at some point in the galaxy's early history, the stars began moving in a coherent, rotating pattern, marking what scientists call the galaxy's spin-up time.
However, the Milky Way did not form in isolation. For decades, scientists have suspected that a violent collision with a smaller galaxy played an important role in shaping the Milky Way as we observe it today. This suspicion was confirmed in 2018, when data from the Gaia mission revealed a large population of stars whose unusual motions could only be explained by a massive merger that occurred about ten billion years ago. This event is now known as the Gaia-Sausage-Enceladus (GSE) merger.
