The Ancient Roots of the Milky Way’s Disks: Evidence for Early Co-Formation Before a Galactic Collision
In a new study led by Lais Borbolato, astronomers explore the early history of our Galaxy to better understand how its two main stellar components—the thin and thick disks—formed. These disks differ in both chemical composition and motion, and for years, scientists have debated whether they formed one after the other or simultaneously. Using detailed information from multiple astronomical surveys, the authors present new evidence that both disks may have begun forming over 11 billion years ago, long before a major galactic merger thought to shape the modern Milky Way.
Stellar Clues from Modern Surveys
To investigate this idea, the team used data from the APOGEE and LAMOST spectroscopic surveys, which measure the chemical makeup of stars, and combined it with precise stellar motion and distance data from the Gaia mission. They then applied the StarHorse code, a Bayesian tool that estimates stellar ages and distances by comparing each star’s observed properties to theoretical models. Their analysis focused on stars in the solar neighborhood and carefully selected those with small uncertainties in their measurements to ensure reliable results.
Chemical Signatures of Two Populations
By plotting the abundance of magnesium relative to iron ([Mg/Fe]) against overall metallicity ([Fe/H]), the authors separated stars into two groups: one with high [Mg/Fe] ratios (typical of the thick disk) and another with lower ratios (typical of the thin disk). Remarkably, they found that a substantial fraction of stars with low [Mg/Fe]—characteristic of the thin disk—were older than 11 billion years. This finding contradicts traditional models that assume the thin disk formed only after the thick disk and implies that the two may have co-existed from early on.
The Role of Stellar Motion
To further test whether these old thin disk stars were truly distinct from thick disk stars, the researchers examined how they move through space. While older thin disk stars were more dynamically “heated” (moving with slightly more randomness), they still had overall motion patterns typical of the thin disk. This supports the idea that they formed in a more turbulent environment than the thin disk stars born later but still belong to the same population.
Galactic Mergers and Star Formation
One of the key questions the study addresses is the timing of the Gaia-Sausage Enceladus (GSE) merger—a major collision between the Milky Way and a smaller galaxy. Many previous theories proposed that this merger triggered the formation of the thin disk. However, since Borbolato and colleagues found thin disk stars that predate the GSE merger, they argue that the thin disk began forming independently. Instead, their results suggest that the merger may have marked the end of thick disk star formation, shifting the Galaxy into a phase dominated by the thin disk.
Challenges to Existing Models
These findings present a challenge to many current models of galactic evolution, which often assume a sequential formation process driven by mergers. The co-existence of chemically distinct populations from the earliest times implies that environmental differences—such as the presence of dense star-forming clumps versus more diffuse gas—may have been enough to create the observed dichotomy in chemical abundances. The GSE merger, while still influential, may not be the sole cause of the thin disk’s birth, but rather a factor that influenced the relative dominance of each disk over time.
A New Picture of Disk Formation
The study concludes that the Milky Way’s thick and thin disks likely formed in parallel, not in sequence, with the early Galaxy giving rise to both populations under different conditions. The GSE merger may have played a key role in stopping the formation of thick disk stars and increasing the rate of thin disk formation. These results call for future simulations of galaxy evolution that can account for both early clump-driven star formation and the effects of later mergers. In doing so, they may offer a more complete picture of how galaxies like our own come to be.
Source: Borbolato
