Galaxies like the Milky Way are thought to grow through mergers with smaller galaxies. However, the precise history of these events remains unclear. Recent research by Lekshmi Thulasidharan and collaborators reveals that the vertical thickness of the Milky Way’s stellar disk holds a record of its merger history. Using stellar age data from surveys like LAMOST and SDSS, and simulations from IllustrisTNG50, this study deciphers how past interactions shaped our galaxy’s structure. The findings point to key merger events, including the significant Gaia-Sausage Enceladus (GSE) event, which occurred over 11 billion years ago, and more recent interactions with the Sagittarius dwarf galaxy.

Data Sources

The researchers relied on stellar age data from two main sources: the LAMOST catalog, which focuses on red giants, and SDSS DR12, which includes metal-poor stars. Ages were calculated using advanced methods like the StarHorse algorithm, which combines observed star properties with stellar evolution models. To ensure accuracy, stars within a specific range of the Milky Way’s disk (8–11 kpc in radius and up to 3 kpc in height) were analyzed. The combined dataset included hundreds of thousands of stars, enabling a detailed examination of the disk’s vertical structure.

Key Results

The study measured the vertical spread of stars, called disk thickness, to uncover patterns in the Milky Way’s evolution. Four significant peaks in disk thickness were identified at 11.13, 5.20, 2.02, and 0.22 billion years ago. These peaks align with major events in the galaxy’s history, such as mergers or close encounters with smaller galaxies. For example, the peak at 11.13 billion years ago corresponds to the GSE merger, which profoundly influenced the galaxy’s structure. More recent peaks are tied to interactions with the Sagittarius dwarf galaxy.

Cross-Validation with Simulations

To confirm their findings, the team compared observations with results from the IllustrisTNG50 simulation, which models galaxy evolution. Simulated Milky Way-like galaxies also showed increased disk thickness during merger events, reinforcing the idea that vertical thickness can trace galactic history. The simulations also suggested that the thick disk formed early in the Milky Way’s history, followed by a transition to the thin disk over billions of years.

Impact of Uncertainty

The researchers accounted for uncertainties in age estimates, which can blur details of ancient events. Even with these challenges, the major patterns remained clear. Simulations revealed that while precise timing may be affected, the general trends, such as the formation of the thick and thin disks, are robust. The study emphasizes the need for even more precise age measurements to resolve older, potentially overlapping merger events.

Conclusion

This research highlights the power of the age-thickness relationship as a tool for uncovering galactic history. By connecting observations with simulations, the team provided new insights into the Milky Way’s formation, from early massive mergers to recent interactions. These methods not only enrich our understanding of our galaxy but also offer a framework for studying others, helping to unravel the mysteries of cosmic evolution.

Source: Thulasidharan