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Enrichment by extragalactic first stars in the Large Magellanic Cloud

Nature Astronomy (2024)Cite this article

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Abstract

The Large Magellanic Cloud (LMC) is the Milky Way’s most massive satellite galaxy, which only recently (~2 billion years ago) fell into our Galaxy. As stellar atmospheres preserve the composition of their natal cloud, the LMC’s recent infall makes its most ancient, metal-deficient (‘low-metallicity’) stars unique windows into early star formation and nucleosynthesis in a formerly distant region of the high-redshift universe. Here we present the elemental abundances of ten stars in the LMC with iron-to-hydrogen ratios ranging from ~1/300th to ~1/12,000th that of the Sun. Our most metal-deficient star is markedly more metal-deficient than any in the LMC with available detailed chemical abundance patterns and was probably enriched by a single extragalactic ‘first-star’ supernova. This star lacks appreciable carbon enhancement, as does our overall sample, unlike the lowest-metallicity stars in the Milky Way. This and other abundance differences affirm that the extragalactic early LMC experienced diverging enrichment processes compared to the early Milky Way. Early element production, driven by the earliest stars, thus, appears to proceed in an environment-dependent manner.

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Fig. 1: Identification of low-metallicity member stars in the LMC.
Fig. 2: Elemental abundance trends of stars in the LMC versus the Milky Way and the Sculptor dwarf galaxy.

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Data availability

The velocities and chemical abundances derived from the long-exposure MIKE spectra in this study are presented in Table 2 and Supplementary Data 1. Abundance measurements for individual absorption features in these stars are provided in Supplementary Data 2. We report short-exposure MIKE and MagE metallicities and carbon abundances in Extended Data Table 2. The stellar spectra of these stars are available from the corresponding author upon request. The proper motions of these stars are available from the Gaia data archive (https://gea.esac.esa.int/archive/). The data tables will be posted in machine-readable format at Zenodo https://doi.org/10.5281/zenodo.10032360 upon publication91. Source data are provided with this paper.

Code availability

The stellar synthesis code MOOG that was used to analyse these data can be retrieved from https://github.com/alexji/moog17scat. The analysis package SMHR that wraps around MOOG can be retrieved from https://github.com/andycasey/smhr. The orbit integration code that includes the Milky Way, LMC and Sagittarius can be retrieved from ref. 20. The Payne4MIKE code used to analyse the short-exposure MIKE spectra can be retrieved from https://github.com/tingyuansen/Payne4MIKE. The chemical abundance analysis of the two MagE spectra was performed using the authors’ implementations of published techniques, which are straightforward to reproduce from the publications, but are available from the corresponding author upon request.

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Acknowledgements

Our data were gathered using the 6.5 m Magellan Baade telescope at Las Campanas Observatory, Chile. A.C. thanks A. Drlica-Wagner, K. Olsen, D. Nidever, G. Stringfellow, W. Cerny, K. Venn, A. Arentsen, V. Placco, I. Roederer, P. Sharda and A. Kravtsov for helpful discussions, and R. Prasad for their support. A.C. also thanks V. Placco for providing a compilation of corrected carbon abundances of metal-poor Milky Way stars. This work benefited from the KICP/UChicago Gaia DR3 sprint and made use of NASAʼs Astrophysics Data System Bibliographic Services, the SIMBAD database (operated at the Strasbourg Astronomical Data Centre, Strasbourg, France) and the open-source Python libraries numpy, scipy, matplotlib and astropy. A.C. is supported by a Brinson Prize Fellowship at the Kavli Institute for Cosmological Physics, University of Chicago. G.L. acknowledges support from the São Paulo Research Foundation (Grant Nos. 2021/10429-0 and 2022/07301-5). A.P.J. acknowledges support from the US National Science Foundation (NSF; Grant Nos. AST-2206264 and AST-2307599). T.S.L. acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (Grant No. RGPIN-2022-04794). K.B. is supported by an NSF astronomy and astrophysics postdoctoral fellowship (Award No. AST-2303858). H.D.A. acknowledges support from the Undergraduate Research Opportunities Program at the Massachusetts Institute of Technology. This work has made use of data from the European Space Agency’s mission Gaia (https://www.cosmos.esa.int/gaia) that has been 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. The national facility capability for SkyMapper has been funded through the Linkage Infrastructure, Equipment and Facilities scheme of the Australian Research Council (Grant No. LE130100104), which was awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. SkyMapper is owned and operated by the Australian National University’s Research School of Astronomy and Astrophysics.

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Authors and Affiliations

  1. Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL, USA

    Anirudh Chiti, Guilherme Limberg & Alexander P. Ji

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

    Anirudh Chiti, Guilherme Limberg & Alexander P. Ji

  3. Department of Physics, Zarqa University, Zarqa, Jordan

    Mohammad Mardini

  4. Jordanian Astronomical Virtual Observatory, Zarqa University, Zarqa, Jordan

    Mohammad Mardini

  5. Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements (JINA-CEE), East Lansing, MI, USA

    Mohammad Mardini, Anna Frebel, Hillary Diane Andales & Kaley Brauer

  6. Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA

    Mohammad Mardini, Anna Frebel & Hillary Diane Andales

  7. Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Astronomia, São Paulo, Brazil

    Guilherme Limberg

  8. Gemini Observatory/NSF’s NOIRLab, La Serena, Chile

    Henrique Reggiani

  9. Department of Physics, University of Wisconsin–Madison, Madison, WI, USA

    Peter Ferguson

  10. Harvard–Smithsonian Center for Astrophysics, Cambridge, MA, USA

    Kaley Brauer

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

    Ting S. Li

  12. Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Toronto, Ontario, Canada

    Ting S. Li

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

    Joshua D. Simon

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Contributions

A.C. designed the technique for generating the Gaia XP metallicity catalogue used in this work, selected candidates for the observations and led the MIKE observations, analysis, interpretation, and paper writing. M.M. generated the Gaia XP metallicity catalogue and assisted with making the MIKE observations and their analysis and interpretation. A.P.J. and G.L. assisted with the analysis, interpretation and paper writing. A.F. assisted with the MIKE observations, interpretation and paper writing. H.R. provided existing MIKE observations of LMC stars and assisted with the interpretation. P.F. generated the catalogue of synthetic Gaia XP photometry. K.B. and H.D.A. led the analysis of the distance of the LMC from the Milky Way when it was producing low-metallicity stars. T.S.L. and J.D.S. led and assisted with the MagE observations. All authors provided feedback on the paper before submission.

Corresponding author

Correspondence to Anirudh Chiti.

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Extended data

Extended Data Fig. 1 Orbit and proper motion analyses of metal-poor LMC stars in this study.

a. The orbit of the LMC and stars observed in this study in Galactocentric X and Z coordinates, when integrated backwards in a potential that includes the Milky Way and the LMC20. The LMC is shown as a thick dashed line, and stars in Table 1 are overplotted. Note that the stars remain bound to the LMC, indicating that they are gravitationally bound members. b. Proper motion vectors with respect to the center of mass motion of the LMC. The proper motions of the stars observed in our study are shown as colored arrows, with the color scheme corresponding to panel (a). The black arrows indicate the average proper motion vector of LMC stars within 0.5° of their spatial location. Note that two of the LMC low metallicity stars are counter-rotating on the plane of the sky with respect to the bulk motion of LMC members, in the center-of-mass frame of the LMC. The center of the LMC is marked by a black cross.

Extended Data Table 1 Summary of our observations of stars in the Large Magellanic Cloud
Full size table
Extended Data Table 2 Properties of low metallicity LMC stars observed for short-durations on MIKE and MagE to solely derive metallicities and carbon abundances
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–3.

Supplementary Data 1

Complete elemental abundances and associated uncertainties from long-exposure, high-resolution Magellan/MIKE spectra. Columns are as follows: the star name; the atomic number and ionization state of the element; the number of features used (N); the solar abundance (Solar); the absolute abundance (Logeps); the chemical abundance scaled by the solar abundance relative to hydrogen ([X/H]); the ratio with respect to the iron abundance ([X/Fe]); the random uncertainty ([X/H]_err) and an upper-limit flag (ul); errors from propagating the uncertainties in each stellar parameter ([X/H]_errteff, [X/H]_errlogg, [X/H]_errvt); the cumulative uncertainty from stellar parameters ([X/H]_errsys); and the total uncertainty ([X/H]_errtot). Following this are the same columns but with respect to iron ([X/Fe]_errteff, [X/Fe]_errlogg, [X/Fe]_errvt, [X/Fe]_errsys, [X/Fe]_errtot).

Supplementary Data 2

Chemical abundances from individual absorption lines and molecular bands for LMC stars from long-exposure, high-resolution Magellan/MIKE spectra. The name of the star is followed by the atomic number and ionization of the element measured from the feature. This is followed by the wavelength (in angstroms), the excitation potential, oscillator strength (log gf), equivalent width, derived abundance and a flag to indicate whether the measurement is an upper limit. Abundances derived from syntheses of features are denoted by nan entries for the equivalent width in the table, and abundances of the CH molecular band are indicated by the number 106.0 in the second column.

Source data

Source Data for Fig. 1

Source data for selected stars in Fig. 1.

Source Data for Fig. 2

Source data for Fig. 2.

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Chiti, A., Mardini, M., Limberg, G. et al. Enrichment by extragalactic first stars in the Large Magellanic Cloud. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02223-w

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