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Enrichment by the first stars in a relic dwarf galaxy

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Abstract

Stars that contain trace amounts of elements heavier than helium (that is, ‘metallicity’) preserve the chemical fingerprints of the first stars. In the Milky Way, nearly all of the lowest-metallicity stars show an extreme over-abundance of carbon. The origin of this signature has remained a mystery owing to the lack of observational constraints on the environments in which it originates. Here we present observations of a star in the >10-billion-year-old ultrafaint dwarf galaxy Pictor II, showing the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with >3,000× relative carbon enhancement. The star’s exceptional paucity in iron and calcium make it clearly preserve enrichment from the first stars in a relic dwarf galaxy; Pictor II is one of the smallest, most chemically primordial systems known. This star supports the hypothesis that extreme carbon enhancement results from low-energy supernovae from the first stars, as the yields of energetic supernovae are harder to retain in small-scale environments. This signature of enrichment by the first stars may trace a regime inaccessible to current high-redshift observations, which can hardly detect the initial enrichment of the smallest galaxies.

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Fig. 1: Identification of the distant Pictor II member star, PicII-503.
Fig. 2: Spectroscopic confirmation of PicII-503 as an ultra metal-poor, carbon-enhanced star.
Fig. 3: Diagnostic plots for relative carbon, calcium and iron abundances for PicII-503.

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Data availability

The X-Shooter spectrum of PicII-503, from which all abundances reported in Table 2 are derived, will be publicly accessible from the ESO archive (http://archive.eso.org) on 30 March 2026. The MagE spectrum, which was used for initial confirmation of the star’s low metallicity and radial velocity, is available from the corresponding author upon request. The MAGIC photometric data used for target selection are too large to include with this Article and will be publicly released as part of the MAGIC Survey Data Release 1. The broad-band photometry of PicII-503 is available via DELVE DR2 (https://datalab.noirlab.edu/data/delve), and its proper motion measurements are available in Gaia DR3. As noted, the primary measurements and conclusions of this work are based entirely on the X-Shooter spectroscopy.

Code availability

The stellar synthesis code MOOG and the analysis package Spectroscopy Made Harder that were used to analyse this data are available via GitHub at https://github.com/alexji/moog17scat (ref. 83) and https://github.com/andycasey/smhr (ref. 85). The velocity analysis of the MagE and X-Shooter spectra is from the authors’ implementation of code described in the methods that is straightforward to replicate and is available from the corresponding author upon request.

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Acknowledgements

A.C. is supported by The Brinson Foundation through a Brinson Prize Fellowship grant. The work of V.M.P. and Y.C. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation. W.C. gratefully acknowledges support from a Gruber Science Fellowship at Yale University. D.J.S. acknowledges support from NSF grant no. AST-2508746. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant no. DGE2139841.

This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES Projects has been provided by the US Department of Energy, the US National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute for Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo á Pesquisa do Estado do Rio de Janeiro, Conselho 12 Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey.

The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Enérgeticas, Medioambientales y Tecnológicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciéncies de l’Espai (IEEC/CSIC), the Institut de Física d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, NSF NOIRLab, the University of Nottingham, the Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex and Texas A&M University.

The results are based on observations at NSF Cerro Tololo Inter-American Observatory, NSF NOIRLab (NOIRLab prop. ID 2019A-0305; Principal Investigator: A. Drlica-Wagner, and NOIRLab prop. ID 2023B-646244; Principal Investigator: A. Chiti), which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the US National Science Foundation.

Our spectroscopic data were gathered using the 6.5-m Magellan Baade telescope located at Las Campanas Observatory, Chile and the ESO Very Large Telescope, specifically based on observations collected at the European Southern Observatory under ESO programme 114.28L2.001. This work made use of NASAs Astrophysics Data System Bibliographic Services, the SIMBAD database, operated at CDS, Strasbourg, France.

The DELVE project is partially supported by the NASA Fermi Guest Investigator Program Cycle 9 no. 91201. This work is partially supported by Fermilab LDRD project no. L2019-011. This material is based upon work supported by the National Science Foundation under grant nos. AST-2108168, AST-2108169, AST-2307126 and AST-2407526.

Fermilab is managed by FermiForward Discovery Group, LLC under contract no. 89243024CSC000002 with the US Department of Energy, Office of Science, Office of High Energy Physics. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US Government purposes.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

Author information

Authors and Affiliations

  1. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, USA

    Anirudh Chiti

  2. Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA

    Anirudh Chiti, Alexander P. Ji, Guilherme Limberg & Alex Drlica-Wagner

  3. Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL, USA

    Anirudh Chiti, Alexander P. Ji, Guilherme Limberg & Alex Drlica-Wagner

  4. NSF NOIRLab, Tucson, AZ, USA

    Vinicius M. Placco & Yumi Choi

  5. Department of Astronomy, University of Virginia, Charlottesville, VA, USA

    Andrew B. Pace, Kaia R. Atzberger & Nitya Kallivayalil

  6. NSF-Simons AI Institute for the Sky, Chicago, IL, USA

    Alexander P. Ji & Alex Drlica-Wagner

  7. Department of Astronomy/Steward Observatory, University of Arizona, Tucson, AZ, USA

    Deepthi S. Prabhu & David J. Sand

  8. Department of Astronomy, Yale University, New Haven, CT, USA

    William Cerny

  9. Center for Astrophysics and Space Astronomy, University of Colorado Boulder, Boulder, CO, USA

    Guy S. Stringfellow

  10. Fermi National Accelerator Laboratory, Batavia, IL, USA

    Alex Drlica-Wagner

  11. Department of Physics and Astronomy, University of Tampa, Tampa, FL, USA

    Denija Crnojević

  12. DiRAC Institute, Department of Astronomy, University of Washington, Seattle, WA, USA

    Peter S. Ferguson

  13. Department of Physics, University of Surrey, Guildford, UK

    Noelia E. D. Noël

  14. Institute for Computational Cosmology, Department of Physics, Durham University, Durham, UK

    Alexander H. Riley

  15. Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Lund, Sweden

    Alexander H. Riley

  16. Observatories of the Carnegie Institution for Science, Pasadena, CA, USA

    Joshua D. Simon

  17. Cerro Tololo Inter-American Observatory/NSF NOIRLab, La Serena, Chile

    Alistair R. Walker & A. Katherina Vivas

  18. Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil

    Clecio R. Bom

  19. Instituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile

    Julio A. Carballo-Bello

  20. ASTRAVEO LLC, Gloucester, MA, USA

    David J. James

  21. Applied Materials Inc., Gloucester, MA, USA

    David J. James

  22. NSF NOIRLab, Hilo, HI, USA

    Clara E. Martínez-Vázquez

  23. David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, Toronto, Ontario, Canada

    Gustavo E. Medina

  24. Department of Astronomy and Astrophysics, University of Toronto, Toronto, Ontario, Canada

    Gustavo E. Medina

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  1. Anirudh Chiti
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Contributions

A.C. led the photometering and subsequent processing of the MAGIC data to catalogue generation, photometric metallicity derivation, spectroscopic targeting, MagE observations, derivation of chemical abundances, radial velocity analyses and drafted the interpretation and text. V.M.P. assisted with the chemical abundance analysis, initial interpretation and text. A.B.P. performed the membership analysis and assisted with the initial interpretation and text. A.P.J. assisted with the chemical abundance analysis, helped scope the interpretation, and assisted with the text. D.S.P. assisted with the MagE observations, the initial interpretation and text. W.C., G.L. and G.S.S. assisted with the initial interpretation and text. A.D.W. served as the internal referee within the collaboration. A.R.W. led the acquisition of the CaHK filter on DECam and provided feedback on the text and interpretation. D.J.S., K.R.A., Y.C., J.D.S., A.H.R., N.K, D.C., N.E.D.N. and P.S.F. provided feedback on the text. D.J.J., J.A.C.B., C.R.B., C.E.M.V, A.H.R., G.E.M. and A.K.V substantially contributed to the observations and infrastructure of the MAGIC and DELVE surveys.

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Correspondence to Anirudh Chiti.

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Nature Astronomy thanks Miho Ishigaki and Stefania Salvadori for their contribution to the peer review of this work. Peer reviewer reports are available.

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Chiti, A., Placco, V.M., Pace, A.B. et al. Enrichment by the first stars in a relic dwarf galaxy. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02802-z

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