How the Milky Way’s Disc Survived a Cosmic Collision
Matthew Orkney and collaborators investigate how the Milky Way’s disc of stars formed and managed to survive despite being shaken by an ancient galactic collision. Using advanced computer simulations and comparing them with data from the European Space Agency’s Gaia satellite, the authors explore how the Galaxy’s structure was built, and how past mergers influenced its stars.
Introduction: A Galaxy with a Story
The Milky Way is considered a fairly typical large galaxy, sitting between galaxies that are still actively forming stars and those that are more quiet. Astronomers know that galaxies like ours grow by pulling in smaller galaxies through a process called hierarchical growth. Evidence from Gaia revealed that our Galaxy collided with another system about 10 billion years ago, an event known as the Gaia-Sausage-Enceladus (GSE). This collision left behind stellar debris and altered the structure of the Galaxy. The authors are especially interested in how this and other mergers shaped the spinning disc of stars where we live today.
Methods: Simulating the Past
To explore these questions, the team used the Auriga simulations, a suite of computer models that follow the formation of Milky Way–like galaxies over cosmic time. These simulations include the physics of star formation, supernova explosions, and black hole activity. From them, the authors identified stellar populations connected to mergers, such as the Aurora stars (the Galaxy’s earliest stars), the Splash stars (disc stars kicked up by collisions), and the starburst populations (new stars formed in a burst during a merger). They then compared the simulated histories to observed data on stellar ages, metallicities (a measure of chemical content), and motions.
Results: Collisions and Consequences
The simulations show that mergers disrupt the stellar disc, often erasing signs of its early formation. In cases of massive collisions, up to half of the disc stars could be “splashed” into halo-like orbits. Still, the disc tends to reform after the event. Importantly, the timing of when the disc began spinning up into its modern form, what the authors call “disc spin-up”, is tricky to pin down. In simulations, this spin-up often appears delayed by mergers, but observations of the Milky Way suggest that disc stars were already rotating by 11 billion years ago. This means the real GSE collision must have been smaller (a minor merger) than some models propose.
Comparing with the Milky Way
Data from Gaia reveal that even the oldest stars in the Milky Way’s disc show significant rotation, suggesting the Galaxy was already organized before the GSE event. The authors highlight that the merger likely had a mass ratio no larger than about 1:7 (the Milky Way being much more massive). They also note that globular clusters, ancient dense star clusters, formed in a burst around 11 billion years ago, matching the timing of the GSE’s first close passage. This is a remarkable link between simulations and real observations.
Discussion: Untangling the Populations
The study finds that populations like Aurora, Splash, and starburst stars overlap in many of their chemical and orbital properties, making it hard to distinguish them in real data. However, one thing is clear: the Milky Way’s disc proved resilient. Despite being battered by an early merger, the disc reformed and survived, allowing our Galaxy to maintain its recognizable structure.
Conclusion: A Survivor Galaxy
This work shows how combining simulations with observations can clarify the Milky Way’s history. Orkney and colleagues argue that the Galaxy’s disc likely spun up around the same time as the GSE collision, roughly 11 billion years ago, and that the merger itself was less violent than once thought. The Milky Way’s story is therefore one of survival: even in the face of cosmic chaos, its disc endured.
Source: Orkney
