In a recent article published in the journal Astronomy & Astrophysics, researchers introduced a method that uses the chemical composition of stars, their "chemical fingerprints", to reveal hidden Galactic structures, particularly focusing on the inner Milky Way. This approach offers a potential new tool for mapping spiral galaxies in the future.
Image Credit: Hector Casasus Hernandez/Shutterstock.com
Observing the Milky Way is Hard
Mapping the Milky Way is complicated because, unlike external galaxies, it must be studied from within. The authors like this challenge to stand in the middle of a brightly lit city at night: while looking toward the outskirts offers a clear view, looking toward the city center results in a blur of overlapping lights and complexity. The Milky Way is a spiral galaxy featuring spiral arms (curved regions of gas, dust, and young stars) whose numbers and shape are hard to determine due to our vantage point. Traditional mapping methods, such as stellar density or gas distribution, only provide a partial view, especially for the inner spiral arms located between the Sun and the Galactic center, which are difficult to trace due to dense dust obscuring the view. This difficulty prompted researchers to explore an alternative idea: mapping the spiral arms by analyzing the chemical elements contained within the stars themselves, rather than tracking their brightness or motion.
The Current Study
To test the idea of chemical mapping, the international team used high-quality spectroscopic data to determine the chemical composition of stars by analyzing the light they emit. The study utilized data from the Gaia-ESO Survey, a significant public spectroscopic project carried out using the Very Large Telescope in Paranal, Chile. This research was informed by previous studies based on the Gaia mission and the APOGEE survey, both of which provide detailed information about stellar chemistry.
The underlying principle of the method is that stars function as "chemical time capsules," preserving the elemental makeup of the gas clouds from which they originated. These gas clouds were enriched by elements scattered by previous stellar generations, such as iron and magnesium released during supernovae explosions. By creating spatial maps based on the ratios of specific chemical elements, the astronomers could trace star formation patterns and the overall chemical evolution of the Milky Way. The study specifically focused on detecting small differences in chemical abundances, a technique which allowed the spiral arms to emerge clearly. Expertise in stellar spectroscopy, Galactic archaeology, and chemical evolution modeling was combined to strengthen the analysis.
What Did They Find?
The chemical method yielded striking results, revealing patterns that were invisible in traditional maps. By mapping the chemical patterns, the team successfully identified regions with specific elemental combinations that align with the known locations of the Scutum and Sagittarius spiral arms. Furthermore, the study detected a chemical feature (a spur) that links these two arms, suggesting a more complex and intricate Galactic structure than previously understood. Dr. C. Viscasillas Vázquez suggests that the arms might have several branches, resembling how rivers divide into tributaries or highways branch into secondary roads.
The team compared their results with recent two-dimensional chemical evolution models developed by Dr. Emanuele Spitoni and colleagues, confirming that the spiral arms influence the chemical makeup of the Milky Way. Dr. Spitoni explained that in their model, the arms rotate at different speeds, boosting star formation as they pass through the disc, which is reflected in variations in the distribution of chemical elements. This process means that spiral arms enhance star formation and leave detectable chemical signatures behind. This result highlights a close link between the dynamics and chemistry of stellar populations. The study complements earlier work that traced outer spiral structures using chemical data by peering deeper into the inner Galaxy, a region where observation is challenging due to dust extinction.
Conclusion
This research demonstrates that the Milky Way's hidden spiral arms can be effectively revealed using a new approach: analyzing the chemical information preserved in stars. This method successfully uncovered structures difficult to detect using traditional means. Dr. C. Viscasillas Vázquez concludes that the technique, which traces the "invisible architecture" of the Galaxy through elemental fingerprints, offers a new perspective on an age-old question. With future surveys providing greater coverage and precision, the use of chemical abundances is poised to become an essential component of Galactic cartography.
Journal Reference
Viscasillas Vázquez, C., et al. (2025). The Galactic inner spiral arms revealed by the Gaia ESO Survey chemical abundances. Astronomy & Astrophysics 698, A91. DOI: 10.1051/0004-6361/202554206, https://www.aanda.org/articles/aa/full_html/2025/06/aa54206-25/aa54206-25.html