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# A relic from a past merger event in the Large Magellanic Cloud

## Abstract

According to the standard cosmological scenario, the large galaxies that we observe today have reached their current mass via mergers with smaller galaxy satellites1. This hierarchical process is expected to take place on smaller scales for the satellites themselves, which should build up from the accretion of smaller building blocks2. The best chance we have to test this prediction is by looking at the most massive satellite of the Milky Way: the Large Magellanic Cloud (LMC). Smaller galaxies have been revealed to orbit around the LMC3,4, but so far the only evidence for mutual interactions is related to the orbital interplay with the nearby Small Magellanic Cloud, which is the most massive LMC satellite. In this work, we report the likely discovery of a past merger event that the LMC experienced with a galaxy with a low star formation efficiency and likely a stellar mass similar to those of dwarf spheroidal galaxies. This former LMC satellite has now completely dissolved, depositing the old globular cluster NGC 2005 as part of its debris. This globular cluster, the only surviving witness of this ancient merger event, is recognizable through its peculiar chemical composition. This discovery is observational evidence that the process of hierarchical assembly has worked also in shaping our closest satellites.

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## Data availability

Most of the data used in this work are available in the public archive of the European Southern Observatory (http://archive.eso.org/eso/eso_archive_main.html and http://archive.eso.org/wdb/wdb/adp/phase3_main/form). All the data are available from the corresponding author upon reasonable request.

## Code availability

The codes used for the chemical analysis are publicly available: GALA (http://www.cosmic-lab.eu/gala/gala.php), ATLAS9 (https://wwwuser.oats.inaf.it/castelli/sources/atlas9codes.html), SYNTHE (https://wwwuser.oats.inaf.it/castelli/sources/synthe.html) and DAOSPEC (http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/STETSON/daospec/). We opt not to make the code used for the chemical evolution modelling publicly available because it is an important asset in the researchers’ tool kits.

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## Acknowledgements

A. Mucciarelli, F.R.F. and L.O. acknowledge the financial support of the project ‘Light-on-Dark’, granted by the Italian MIUR through contract PRIN-2017K7REXT. D.R. gratefully acknowledges the International Space Science Institute (ISSI) in Bern, Switzerland, and the International Space Science Institute Beijing (ISSI-BJ) in Beijing, China, for support provided to the team ‘Chemical abundances in the ISM: the litmus test of stellar IMF variations in galaxies across cosmic time’. D.R. and M.B. acknowledge the financial support of INAF through the Main Stream grant CRA 1.05.01.86.28 assigned to the project ‘SSH: the Smallest Scale of Hierarchy’. A. Mucciarelli thanks J. Johnson for sharing the MIKE spectra. A. Mucciarelli dedicates this work to the memory of his twin brother Simone.

## Author information

Authors

### Contributions

A. Mucciarelli designed the study, coordinated the work and performed the data analysis. D.M. led the scientific interpretation. A. Minelli contributed to the spectroscopic analysis. D.R. computed the chemical evolution models and contributed to the scientific interpretation. M.B. contributed to the scientific interpretation and to the writing. F.R.F., F.M. and L.O. contributed to the presentation of the paper. All the authors critically contributed to the work presented here.

### Corresponding author

Correspondence to A. Mucciarelli.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Andres Piatti and the other, anonymous, reviewers for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–5, Tables 1–3 and references.

### Supplementary Data 1

Machine-readable version of Supplementary Table 3. The table lists for the individual LMC globular cluster stars the effective temperature, the logarithm of the surface gravity, the microturbulent velocity, and the iron abundance with the corresponding error calculated as the sum in quadrature of the statistical error and the uncertainty arising from the atmospheric parameters.

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Mucciarelli, A., Massari, D., Minelli, A. et al. A relic from a past merger event in the Large Magellanic Cloud. Nat Astron 5, 1247–1254 (2021). https://doi.org/10.1038/s41550-021-01493-y

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• DOI: https://doi.org/10.1038/s41550-021-01493-y