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A stellar stream remnant of a globular cluster below the metallicity floor

Abstract

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early Universe1. Although a few hundred stars with metal content below 1,000th of the solar iron content are known in the Galaxy2,3,4, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least approximately 0.2% of the solar metallicity \(([{\rm{Fe}}/{\rm{H}}]\gtrsim -2.7)\). This metallicity floor appears universal5,6, and it has been proposed that protogalaxies that merged into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day7. Here we report observations of a stellar stream, C-19, whose metallicity is less than 0.05% of the solar metallicity \(([{\rm{F}}{\rm{e}}/{\rm{H}}]=-3.38\pm 0.06\,({\rm{s}}{\rm{t}}{\rm{a}}{\rm{t}}{\rm{i}}{\rm{s}}{\rm{t}}{\rm{i}}{\rm{c}}{\rm{a}}{\rm{l}})\pm 0.20\,({\rm{s}}{\rm{y}}{\rm{s}}{\rm{t}}{\rm{e}}{\rm{m}}{\rm{a}}{\rm{t}}{\rm{i}}{\rm{c}}))\). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and is significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.

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Fig. 1: Properties of the C-19 member stars.
Fig. 2: Metallicity properties of C-19 and its stars observed by spectroscopy.
Fig. 3: High-resolution GRACES spectra for three members of C-19.

Data availability

The data used in this article are listed in Extended Data Tables 15.

Code availability

The codes used for the analysis were not designed to be made public but can be requested from the corresponding author.

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Acknowledgements

N.F.M., R.A.I., A.A. and Z.Y. gratefully acknowledge support from the French National Research Agency (ANR)-funded project Pristine (ANR-18-CE31-0017) along with funding from Centre National de la Recherche Scientifique (CNRS)–INSU through the Programme National de Cosmologie et Galaxies and through CNRS grant PICS07708 and from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 834148). K.A.V. is grateful for funding through the National Science and Engineering Research Council of Canada (NSERC) Discovery Grants and CREATE programmes. E.S. acknowledges funding through VIDI grant ‘Pushing Galactic Archaeology to its Limits’ (project number VI.Vidi.193.093), which is funded by the Dutch Research Council (NWO). J.I.G.H. acknowledges financial support from Spanish Ministry of Science and Innovation (MICINN) project AYA2017-86389-P, and also from the Spanish MICINN under the 2013 Ramón y Cajal programme (RYC-2013-14875). G.F.T. acknowledges support from the Agencia Estatal de Investigación of the Spanish MCINN (grant number FJC2018-037323-I). We thank the Canada–France–Hawaii Telescope (CFHT) staff for performing the Pristine observations in queue mode, for their reactivity in adapting the schedule and for answering questions during the data-reduction process. We are also grateful to the High Performance Computing Centre of the University of Strasbourg and its staff for very generous time allocation and for their support during the development of the STREAMFINDER project. This research used the SIMBAD database70, managed and run  at CDS, Strasbourg, France. This research used the VizieR catalogue access tool71, CDS, Strasbourg, France. This work is based on observations obtained with MegaPrime–MegaCam, a joint project of the CFHT and CEA–DAPNIA, at the CFHT, which is operated by the National Research Council of Canada, the Institut National des Science de l’Univers of the French CNRS and the University of Hawaii. ESPaDOnS is a collaborative project funded by France (CNRS, MENESR, OMP, and LATT), Canada (NSERC), the CFHT and the European Space Agency. Data were reduced with use of the CFHT-developed OPERA data-reduction pipeline. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the Indigenous Hawaiian community. We are very fortunate to have had the opportunity to conduct observations from this mountain. This work is based on observations obtained with Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES), as part of the Gemini Large and Long Program, GN-2020B-LP-102. The international Gemini Observatory, a program of NSF’s NOIRLab, is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation, on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil) and Korea Astronomy and Space Science Institute (Republic of Korea). This work is based on observations made with the GTC telescope, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, under Director’s Discretionary Time. This work was partly based on data obtained with the instrument OSIRIS, built by a consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government. This work used data from the European Space Agency 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. Guo Shoujing Telescope (LAMOST) is a national major scientific project of the Chinese Academy of Sciences. Funding for the project was provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.

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Authors and Affiliations

Authors

Contributions

N.F.M. is a co-leader of the Pristine survey, led the discovery of the C-19 stream, coordinated the spectroscopic observations and co-led the writing of the manuscript. K.A.V. led the GRACES spectroscopic follow-up and the analysis of the resulting spectra, and co-led the writing of the manuscript. D.S.A. led the analysis of the OSIRIS spectra and the writing of the corresponding section of the manuscript. E.S. is a co-leader of the Pristine survey and derived the Pristine photometric metallicities. J.I.G.H. coordinated the OSIRIS follow-up, performed the radial velocity analysis of these spectra and was greatly involved in writing this part of the manuscript. R.A.I. led the STREAMFINDER analysis and derived the orbit of the stream. P.B., E.C. and F.S. derived stellar and orbital parameters for the stars with spectroscopic follow-up. All other authors helped in the development of the Pristine survey and all authors assisted in the development and writing of the paper. A.M. derived the relations used to infer the stellar parameters of the spectroscopically observed stars.

Corresponding author

Correspondence to Nicolas F. Martin.

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Extended data figures and tables

Extended Data Fig. 1 Pristine photometric information for all stars of the C-19 stream selected by STREAMFINDER and present in the Pristine survey.

Large symbols represent stars with G0 < 19.0, for which the STREAMFINDER selection is very reliable, and small symbols represent stars fainter than this limit, which are more likely contaminated in the STREAMFINDER catalogue (see also Fig. 1). The lines represent model expectations as determined from the spectral libraries and filter response curves, without an assumption on whether the star is a dwarf or a giant10. Both model lines and data points are color-coded by their \({[{\rm{Fe}}/{\rm{H}}]}_{{\rm{Pristine}}}\) metallicities. Most C-19 candidate members are located in the region that corresponds to metallicities below \([{\rm{Fe}}/{\rm{H}}]=-3.0\) (above the blue line). For the large data points, we specifically used a photometric metallicity model tailored to giant stars. Near the tip of the red giant branch, it deviates significantly from the generic model represented by the coloured lines and explains the higher metallicity of the right-most point compared with the models.

Extended Data Fig. 2 Favourite orbital solutions for the C-19 stream.

The dashed line shows the orbit of C-19 constrained using the proper motions (red symbols in a, b) of C-19 members identified by STREAMFINDER, their Gaia parallaxes (c), their radial velocities when available (d) and their location on the sky compared with the distribution of extinction45 (e). The orbit determined without using the Gaia parallax information but instead anchoring the distance at 18 kpc is represented by the dotted line. f, g The two orbits, integrated for ±1 Gyr, projected on the Galactic plane, and in the Rz plane. The thick red lines correspond to the part of the orbits that overlaps the observed C-19 member stars.

Extended Data Fig. 3 Spectra of the C-19 member stars observed with OSIRIS, normalized using a running mean filter after removing the velocity signal in the rest frame (black lines), together with the best fit (blue lines) derived by adopting a fitting procedure.

The metallicity, [Fe/H], computed from [M/H] and [Ca/H] is also indicated for each target (see the text for more details).

Extended Data Table 1 List of potential C-19 members from the STREAMFINDER sample
Extended Data Table 2 Summary of observations for the C-19 candidate stars
Extended Data Table 3 Spectroscopic parameters and one-dimensional LTE chemical abundances for the Gemini–GRACES spectra
Extended Data Table 4 Spectral lines and atomic data used for the chemical abundances for the Gemini–GRACES spectra
Extended Data Table 5 Stellar parameters and abundances of C-19 stars observed with GTC–OSIRIS

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Martin, N.F., Venn, K.A., Aguado, D.S. et al. A stellar stream remnant of a globular cluster below the metallicity floor. Nature 601, 45–48 (2022). https://doi.org/10.1038/s41586-021-04162-2

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