Unraveling the Milky Way’s Past: Tagging Stellar Substructures with Chemistry and Motion
The Milky Way’s halo is a fossil record of our Galaxy’s long and violent history. Over billions of years, small galaxies and star clusters have merged into the Milky Way, leaving behind remnants in the form of stellar streams, globular clusters, and other substructures. In this study, Kristopher Youakim and Karin Lind from Stockholm University introduce a new method, chemo-kinematic tagging, to connect these remnants by using both the chemical makeup and motion of stars.
Tracing the Galactic Family Tree
Stars that form together share similar chemical compositions and move through space along related orbits. Youakim and Lind combine these two properties to group stars that might share a common origin. This approach improves upon earlier methods that relied only on motion (“kinematic tagging”) or chemistry (“chemical tagging”). Using data from large surveys such as Gaia and GALAH, they measured how 5,347 stars from 229 known Milky Way substructures move and what elements they contain, especially their [Fe/H] values, a measure of how metal-rich or metal-poor the stars are.
A Map of the Halo’s Hidden Structures
To organize this massive dataset, the team used a machine learning tool called t-distributed stochastic neighbor embedding (t-SNE), which compresses complex, multi-dimensional data into a 2D “map.” This allowed them to visually identify clusters of stars, globular clusters, and streams that occupy similar regions in this “chemo-kinematic” space. Each cluster in the map potentially represents stars that came from the same ancient galaxy or star-forming region.
Rediscovering Known Mergers and Finding New Links
The algorithm successfully recovered many previously known large-scale merger events, including the Gaia-Sausage-Enceladus (GSE), Sequoia, Thamnos, and LMS-1/Wukong structures. It also identified connections between these major events and specific globular clusters, supporting earlier studies that suggest about 44% of Milky Way globular clusters were accreted from other galaxies. Among smaller-scale findings, the researchers confirmed a close relationship between the Orphan-Chenab stream and the Grus II ultra-faint dwarf galaxy, evidence that both were likely absorbed in the same galactic merger.
Building a Coherent Picture of Galactic Evolution
By comparing how the identified groups appear in both their orbital and chemical properties, Youakim and Lind validated that each represents a physically meaningful structure. Some groups, such as those linked to the “Splashed Disc,” likely formed within the Milky Way but were later disturbed by ancient mergers. Others, like GSE and Sequoia, were external galaxies entirely consumed by the Milky Way’s gravity. Together, these results show how mergers of different sizes have collectively built up the Galaxy’s stellar halo over cosmic time.
The Promise of Chemo-Kinematic Tagging
This work demonstrates the power of combining data-driven techniques with precise stellar measurements to piece together the Milky Way’s assembly history. While much remains to be explored, particularly in refining how chemical patterns distinguish between overlapping groups, Youakim’s chemo-kinematic approach offers a new pathway toward reconstructing the Galaxy’s ancient mergers in unprecedented detail. As data from Gaia and future surveys continue to improve, astronomers will be able to map the Milky Way’s “family tree” with increasing clarity, star by star.
Source: Youakim
