TREASURES: Digging into dwarf galaxies

Project Information

TREASURES

Grant agreement ID: 101117455

DOI

10.3030/101117455

EC signature date 18 September 2023

Start date 1 January 2024

End date 31 December 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

UNIVERSITA DEGLI STUDI DI FIRENZE
Italy

Periodic Reporting for period 1 - TREASURES (TREASURES: Digging into dwarf galaxies)

Reporting period: 2024-01-01 to 2026-06-30

Summary of the context and overall objectives of the project

The Milky Way is an environment rich with satellite galaxies, stellar streams and accreted systems. Encoded in these structures is the formation and evolution of the Galaxy through cosmic time. The goal of TREASURES is to unlock these troves of hidden information by exploiting a new groundbreaking spectroscopic survey, 4DWARFS (4MOST survey of Dwarf Galaxies and Their Stellar Streams). This 5-year survey will collect high-quality spectra of individual stars in
over 40 dwarf galaxies, and dozens of stellar streams from systems currently undergoing disruption. TREASURES will provide the community with stellar ages, radial velocities and chemical abundances for over 100,000 stars, and thus increase the number of stars with detailed abundance information in dwarf galaxies and streams by orders of magnitude. Furthermore, TREASURES will take advantage of additional complementary spectra, and will use state-of-the-art models to contrast these new large data against theory.

[WP0] DATA - Acquisition and Analysis: Successfully executing the 4DWARFS survey, acquire additional complementary data, and complete the analysis necessary for the scientific goals of TREASURES.
[WP1] ORIGIN - First stars: What are the properties of the first stars in the Universe? TREASURE will significantly increase the number of known metal-poor stars, and provide analysis of newly discovered unique stellar objects. Thus, constraining the properties of the first stars.
[WP2] EVOLUTION - Nucleosynthesis: How are the chemical elements created and distributed? TREASURES will investigate the detailed chemical abundance patterns of stars, and contrast against models, with the aim of constraining the properties of their nucleosynthetic sites (SN, SNIa, AGB stars, neutron star mergers).
[WP3] GLOBAL PICTURE - Hierarchical Galaxy Formation: What was the hierarchical buildup of the Milky Way? What is the structure of dark matter in different systems? This WP will provide homogeneous radial velocities, chemical abundances and stellar ages for over 100,000 stars, in over 40 galaxies and dozens of stellar streams. It will also give constraints on binary properties and determine the structure of dark matter haloes (core vs cusp). Finally, this will allow to quantify and characterize hierarchical galaxy formation down to the smallest mass scales.

TREASURES will thus paint a comprehensive picture of the formation and evolution of the Milky Wa environment, from cosmic dawn until the present day.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

[WP0] The TREASURES team has contributed in various way to the success of the 4MOST collaboration, such as Chairing the Science Coordination Board; contributing to the Commissioning and Science Verification Phases, and the 4MOST abundance pipeline. Finally the TREASURES team has optimized and finalized the target selection for the 4DWARFS survey.

[WP0] We have developed and published an online tool to apply NLTE corrections for large databases of metal-poor stars (Koutsouridou et al. 2025). These corrections are fundamental to the main targets of 4DWARFS, and will play a key role in the successful outcome of TREASURES.

[WP0] Successful observational proposals: 1) “The number of First Supernovae in the faintest galaxies” (ESO VLT, 26h); as well as several proposals for the use of 4MOST supplementary fibers: 2) “Search for GSE members” (596444 stars); 3) “4MOST at the forefront of stellar stream research” (78078); 4) “Extremes of the Sagittarius dwarf spheroidal galaxy” (63006); 5) “Boosting the Faintest Galaxies in 4MOST” (4454); 6) “Li-rich giants” (30677); 7) “AGB Stars and their Companions” (7350).

[WP1] We have further developed and enhanced an available chemical evolution model of the earliest chemical enrichment to now account for all elements from C to Zn (previously only C and Fe), and add more nucleosynthetic sources (SNIa and AGB stars). Now we have an in-house code to interpret the 4DWARFS data (Koutsouridou et al. 2025; and 2026 in prep).

[WP1] Analyzing two poorly studied ultra faint dwarf galaxies (Verdiani et al. 2026, in press), led to the discovery of 5 stars which are descendants of the first stars. We have analyzed a very unique descendant of the first stars (Cuadra et al. to be submitted in spring 2026), published a very unique first star descendant in Skúladóttir et al. 2024, and produced results on modeling a metal-poor dwarf galaxy in Rossi et al. 2025.

[WP2] The TREASURES team has worked on the investigation of the mysterious formation of heavy elements. One paper has been published with collaborators (Lombardo et al. 2025), while two more are planned for 2026, both lead by PDs of TREASURES.

[WP3] With collaborators we have extensively studied the last major merger of the Milky Way (Ernandes et al. 2024, 2025; Buder et al. 2026a, 2026b submitted). In particular, we have shown the first observational evidence that this merger experienced at least two passages before being dispersed through the Milky Way (Skúladóttir et al. 2025).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

A key accomplishment is TREASURES’ participation in the incredibly successful commissioning of the 4MOST instrument, which will take all the data for the 4DWARFS survey. This instrument is one-of-a-kind, with a novel and complex design, 2400 fibers, both in high-resolution (800 fibres, R~20,000), and low-resolution (1600 fibres , R~6,000). A successful installment of an instrument like this is not guaranteed and naturally this was the accomplishment of the entire 4MOST team. TREASURES contributed significantly with a dedicated Postdoc who participated in over year of training, assisted in person on the mountain, and completed many tests to ensure that the quality of the spectra was sufficient for the TREASURE science, as well as assisting with the final reporting.

Some of the main accomplishments of the first 2 years of TREASURES is the completion of the very complex target selection of 4DWARFS, and the finalizing of the survey design. This survey will be one of its kind and go far beyond the status quo. In addition, the TREASURE teams designed and got approved 6 individual supplementary surveys to 4DWARFS, totaling around 780,000 stars, from which we can expect over 150,000 to be observed. This will mean a huge jump in the scientific impact of TREASURES, compared to what was originally envisioned.

A major milestone was the development of a first-of-its kind online tool that allows for the rapid calculation of NLTE corrections for metal-poor stars (Koutsouridou et al. 2024). This is fundamental in the era of large data, and allows to quickly compare the effects of departure from LTE, across all of the available models in the literature.

Finally for the first time we discovered a clear observational evidence that the last major merger of the Milky Way experienced at least two passages before being dispersed (Skúladóttir et al. 2025). This has long be predicted by theory, but was now confirmed with unique chemical tracers measured through high-quality analysis. This inspired immediate follow-up theoretical investigations (Buder 2026a,b, submitted to MNRAS), as well as the successful 4MOST supplementary target proposal “Search for GSE members using 4MOST" (596444 stars), which will even further enhance the impact of TREASURES.