# Thick-to-thin disc transition and gas disc shrinking induced by the Gaia-Sausage-Enceladus merger

**Authors:** Natsuki Funakoshi, Daisuke Kawata, Jason L. Sanders, Ioana Ciucă, Robert J. J. Grand, HanYuan Zhāng  
**arXiv:** [2507.22979](https://arxiv.org/abs/2507.22979)  
**Date:** 2025-07-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2507.22979v1/

## Key points

- Researchers studied Milky Way disc formation by analyzing structural and kinematic properties related to stellar age
- Data from red giant stars from APOGEE DR17 and Gaia DR3 used to model age-dependent stellar kinematics
- Significant transition in Milky Way disc population around 10 billion years ago identified, with older stars showing short scale lengths (thick disc) and younger stars showing increasing scale length (thin disc)
- Transition linked to Gaia-Sausage-Enceladus merger triggering a starburst event
- Stars formed around 10 billion years ago displayed a dip in scale length shorter than thick disc, attributed to gas disc shrinking from cold to hot gas accretion modes
- Proposed scenario for disc formation involves thick disc forming under cold-mode accretion, GSE merger inducing starburst and transition to hot-mode accretion, temporary shrinking of star-forming gas disc due to rapid gas consumption, growth of thin disc occurring inside-out through hot-mode smooth gas accretion
- Previous literature suggests bimodality in [𝛼/Fe]–[Fe/H] distributions among solar-neighbourhood stars may be explained by star formation quenching in Galactic discs due to delays between episodes of gas accretion
- New insights provide valuable context for understanding complex processes involved in Milky Way evolution over billions of years

## Summary

In a recent study, researchers have delved deeper into the formation of the Milky Way disc by examining its structural and kinematic properties as they relate to stellar age. By analyzing data from red giant stars obtained from APOGEE DR17 and Gaia DR3, the team was able to model age-dependent stellar kinematics using a quasi-isothermal distribution function. Through the use of non-parametric splines, they were able to fit disc parameters based on age and identify a significant transition in the population of the Milky Way disc that occurred approximately 10 billion years ago. The findings suggest that stars older than this transition point exhibit short scale lengths, characteristic of what is known as the thick disc. In contrast, younger stars show an increasing scale length with decreasing age, indicating an inside-out formation process for what is referred to as the thin disc. This transition aligns with the conclusion of a starburst event triggered by the merger known as Gaia-Sausage-Enceladus (GSE). Interestingly, stars formed around 10 billion years ago displayed a dip in scale length even shorter than that of the thick disc. Comparison with simulations from Auriga suggests that this dip may be attributed to gas disc shrinking resulting from a shift from cold to hot gas accretion modes. The proposed scenario for disc formation involves several key stages: first, the thick disc forms under cold-mode accretion; secondly, the GSE merger induces a starburst and increases the total mass of the Galaxy, leading to a transition to hot-mode accretion; thirdly, rapid gas consumption causes temporary shrinking of the star-forming gas disc; finally, growth of the thin disc occurs in an inside-out manner as the size of the star-forming gas disc expands through hot-mode smooth gas accretion. Moreover, previous literature has suggested a potential explanation for observed bimodality in [𝛼/Fe]–[Fe/H] distributions among solar-neighbourhood stars through a concept known as star formation quenching in Galactic discs. This theory posits that there may have been periods where star formation was temporarily suppressed due to delays between episodes of gas accretion. Overall, these new insights shed light on complex processes involved in Milky Way evolution and provide valuable context for understanding how our galaxy's structure has evolved over billions of years.