Astronomers have long sought to determine the formation date of the Milky Way's disk. A key clue lies in the motions and ages of stars: at some point in the Galaxy's early history, stars began to move in a coherent rotation, marking what scientists call the 'initial rotation phase' of the Galaxy.
However, the Milky Way did not form in isolation. For decades, scientists have suspected that a violent collision with a smaller galaxy played a major role in shaping the Milky Way as we know it today.
This hypothesis 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 10 billion years ago.
This event is now known as the Gaia-Sausage-Enceladus (GSE) merger.
From lessons learned in simulations, the authors deduce that the latter probably occurred about 11 billion years ago, earlier than many previous estimates, a period that coincides with a sharp increase in star cluster formation in the Milky Way. This likely triggered a veritable galactic fireworks display, stimulating star formation and promoting the formation of globular clusters.
The results of this study highlight the fundamental link between galactic structure and ancient collisions, two phenomena that must be understood together to unravel the history of our Galaxy. Scientists hope that new data from the James Webb Space Telescope and the ALMA array will allow them to observe the formation of similar galaxies in the distant universe and better understand the processes that shaped our own galaxy.
Here we see the evolution of a galaxy, from its initial building blocks to the final spiral disk, an image taken from the Auriga simulation suite.
The main image shows the combined density of gas and dark matter, with gas colored according to its ambient temperature (blue corresponding to colder temperatures and red to warmer temperatures). The circular insets show the stars of the central galaxy, oriented both face-on and edge-on relative to the disk's angular momentum. These images are obtained by combining the K, B, and U stellar photometry filters.