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# A new class of fossil fragments from the hierarchical assembly of the Galactic bulge

## Abstract

The formation and evolutionary processes of galaxy bulges are still unclear, and the presence of young stars in the bulge of the Milky Way is largely debated. We recently demonstrated that Terzan 5, in the Galactic bulge, is a complex stellar system hosting stars with very different ages and a striking chemical similarity to the field population. This indicates that its progenitor was probably one of the giant structures that are thought to generate bulges through coalescence. Here we show that another globular cluster-like system in the bulge (Liller 1) hosts two distinct stellar populations with remarkably different ages: only 1–3 Gyr for the youngest, and 12 Gyr for the oldest, which is impressively similar to the old component of Terzan 5. This discovery classifies Liller 1 and Terzan 5 as sites of recent star formation in the Galactic bulge and provides clear observational proof that the hierarchical assembly of primordial massive structures contributed to the formation of the Milky Way spheroid.

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## Data availability

The photometric data that support the plots and other findings of this study are available from the corresponding author upon reasonable request. The catalogues are also publicly downloadable from the website of the Cosmic-Lab project (http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). All the HST images are publicly available from the Mikulski Archive for Space Telescopes (http://archive.stsci.edu/).

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## Acknowledgements

This research is part of the project COSMIC-LAB at the Physics and Astronomy Department of the University of Bologna (http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). The research has been funded by project Light-on-Dark, granted by the Italian MIUR through contract PRIN-2017K7REXT. The research is based on data acquired with the NASA/ESA HST under project GO 15231 (principal investigator F.R.F.) at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. It is also based on observations obtained at the GEMINI Observatory. S.S. gratefully acknowledges financial support from the European Research Council (ERC-CoG-646928, Multi-Pop). E.V. acknowledges the Excellence Cluster ORIGINS funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2094 – 390783311. D.G. gratefully acknowledges support from the Chilean Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA) BASAL grant AFB-170002. D.G. also acknowledges financial support from the Dirección de Investigación y Desarrollo de la Universidad de La Serena through the Programa de Incentivo a la Investigación de Académicos (PIA-DIDULS). S.V. gratefully acknowledges the support provided by Fondecyt reg. n. 1170518.

## Author information

Authors

### Contributions

F.R.F. designed the study and coordinated the activity. C.P., C.C., B.L., E.D., A.M., S.S., E.V., F.M., S.V., C.M.B. and G.B. analysed the photometric datasets. E.D. determined the stellar proper motions. F.R.F., C.P. and B.L. wrote the first draft of the paper. E.D., L.O., R.M.R., E.V. and D.G. critically contributed to the paper presentation. All authors contributed to the discussion of the results and provided comments on the various versions of the manuscript.

### Corresponding author

Correspondence to F. R. Ferraro.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 The dataset.

Summary of the photometric dataset used in this work.

### Extended Data Fig. 2 CMDs of the regions beyond the HST-GEMINI field of view.

a,b, CMDs of the area sampled with the parallel WFC3 observations beyond the tidal radius of Liller1, illustrating the distribution of Galactic field stars in the vicinity of the cluster. c, CMD of a 50"x50" region approximately located at 100" from the centre of Liller1, obtained by combining the NICMOS observations retrieved from the Archive with the optical ACS-HST observations presented in this paper.

### Extended Data Fig. 3 Photometric completeness.

The completeness of the sample is shown for three different radial bins from the gravitational centre of Liller 1 (see labels), both in the I-band and in the K-band (panels a,and b, respectively).

### Extended Data Fig. 4 Determining the age of the OP in Liller1.

The value of the χ2 parameter (see Section ‘Measuring the age of the old stellar population’ in Methods) is plotted as a function of the age of seven isochrones (with t=10, 11, 11.5, 12, 12.5, 13, 14 Gyr) computed for the quoted metallicity of the OP ([M/H]=−0.3). The minimum of the χ2 parameter suggests an age of 12 ± 1 Gyr for the OP of Liller 1.

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Ferraro, F.R., Pallanca, C., Lanzoni, B. et al. A new class of fossil fragments from the hierarchical assembly of the Galactic bulge. Nat Astron 5, 311–318 (2021). https://doi.org/10.1038/s41550-020-01267-y

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