Abstract
Investigation of the assembly history of the Milky Way and its closest satellites. Aim of the project is to reconstruct the assembly history of the Milky Way and its satellites, using the chemical abundances of their stars as inferred from high-resolution spectroscopy. An important fraction of the stars populating the Galactic Halo were accreted from disrupted satellited and the use of chemical tagging is a powerful tool to reconstruct the nature of these building block of our galaxy, also with the comparison with other survived close systems. The main goals are: (1) chemical characterization of the stellar populations in three closest Milky Way's satellites, namely the Large and Small Magellanic Clouds, and the Sagittarius dwarf galaxy. (2) chemical description of Galactic stars selected according to their dynamical properties, in particular candidate in-situ and accreted stars; (3) comparison between the chemical properties of accreted stars in the Galctic halo and in the Milky Way's satellites; (4) development of new chemical evolution models to describe the chemical patterns of all the investigated galaxies.
Results achieved
: The LEGO project has successfully achieved its main objective of reconstructing the assembly history of the Milky Way through an innovative chemo-dynamical approach that combines homogeneous high-resolution spectroscopy, Gaia astrometry, and tailored chemical evolution models. By investigating globular clusters, Milky Way field stars, and nearby satellite galaxies as complementary stellar tracers, the project has provided a comprehensive characterization of the Galaxy's building blocks and of the merger events that shaped its evolution. A major outcome of the project has been the homogeneous chemical analysis of a large number of stellar systems. During the project, more than 600 retrograde halo stars, about ten Galactic globular clusters, more than 200 stars in the Small Magellanic Cloud (SMC), over 200 stars in the Sagittarius nuclear star cluster M54, and dozens of stars in the main body of the Sagittarius dwarf galaxy were analyzed. These datasets represent some of the largest and most homogeneous spectroscopic samples currently available for Galactic archaeology. The study of Milky Way globular clusters demonstrated the power of chemical tagging in distinguishing stellar systems formed in different progenitor galaxies. Homogeneous abundance analyses revealed subtle but significant chemical differences between in-situ and accreted clusters, allowing more robust reconstruction of their origins (Ceccarelli et al. 2024a,b). The project also provided new insights into the formation of peculiar stellar systems such as Omega Centauri (Alvarez Garay et al. 2024) and M54 (Alvarez Garay et al. 2026), showing that their chemical properties are consistent with complex formation histories involving globular-cluster mergers. In addition, new dynamical classifications of Galactic globular clusters (De Leo et al. 2026) and the first spectroscopic characterization of several poorly studied clusters significantly improved our knowledge of the Galactic globular cluster system (Ceccarelli et al. 2026). For Milky Way field stars, the project established the large spectroscopic survey A Walk on the Retrograde Side (WRS), which now includes nearly 650 retrograde stars (Ceccarelli et al. 2024a,b, 2025a,b). The homogeneous chemical abundances derived for this sample allowed the characterization of important merger remnants, including Gaia-Enceladus and Thamnos, demonstrating that chemical abundances provide a fundamental complement to orbital information. The project also showed that some structures previously interpreted as merger remnants are instead produced by resonant trapping induced by the Galactic bar, emphasizing the importance of combining dynamics with chemistry when reconstructing the Galaxy's accretion history (De Leo et al. 2025). The investigation of Local Group satellite galaxies provided a new benchmark for interpreting the chemical composition of accreted stellar populations. Detailed abundance analyses of stars in Sagittarius and the SMC revealed distinctive nucleosynthetic signatures that differ significantly from those of Milky Way stars (Liberatori et al. 2025, Anoardo et al. 2026, Santarelli et al. 2026). In particular, the project demonstrated that dwarf galaxies exhibit enhanced r-process enrichment relative to α-elements, establishing abundance ratios such as [Eu/α] as powerful diagnostics to distinguish accreted from in-situ stellar populations. New chemical evolution models successfully reproduced many of the observed abundance trends while highlighting fundamental differences between the chemical evolution of the Milky Way and low-mass galaxies (Palla et al. 2025, 2026). An additional important achievement was the development of new grids of stellar atmosphere models, fluxes, opacity tables, and theoretical colours with improved sampling in stellar parameters and chemical composition. These models have been made publicly available through the KOALA database, (Mucciarelli et al. 2026) providing an important resource for the broader astronomical community and supporting future spectroscopic investigations. Overall, the project produced an exceptionally strong scientific output, with 44 papers published or submitted in leading international peer-reviewed journals, already receiving more than 300 citations. Beyond its direct scientific results, LEGO has established homogeneous spectroscopic datasets, chemical evolution models, and publicly available analysis tools that will serve as fundamental references for upcoming large spectroscopic surveys such as MOONS and 4MOST. The combination of homogeneous chemical abundances, accurate dynamical information, and dedicated theoretical modelling has demonstrated the effectiveness of chemical tagging as a key tool for reconstructing the formation and evolutionary history of the Milky Way and its satellite galaxies.Project details
Unibo Team Leader: Alessio Mucciarelli
Unibo involved Department/s:
Dipartimento di Fisica e Astronomia "Augusto Righi"
Coordinator:
ALMA MATER STUDIORUM - Università di Bologna(Italy)
Total Eu Contribution: Euro (EUR) 187.972,00
Total Unibo Contribution: Euro (EUR) 170.139,00
Project Duration in months: 24
Start Date:
28/09/2023
End Date:
28/02/2026
