Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

# 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.

This is a preview of subscription content, access via your institution

## Access options

\$32.00

All prices are NET prices.

## 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.

## References

1. Frebel, A. & Norris, J. E. Near-field cosmology with extremely metal-poor stars. Ann. Rev. Astron. Astrophys. 53, 631–688 (2015).

2. Yong, D. et al. The most metal-poor stars. III. The metallicity distribution function and carbon-enhanced metal-poor fraction. Astrophys. J. 762, 27 (2013).

3. Li, H., Tan, K. & Zhao, G. A catalog of 10,000 very metal-poor stars from LAMOST DR3. Astrophys. J. Supp. 238, 16 (2018).

4. Aguado, D. S. et al. The Pristine survey - VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates. Mon. Not. R. Astron. Soc. 490, 2241–2253 (2019).

5. Beasley, M. A. et al. An old, metal-poor globular cluster in Sextans A and the metallicity floor of globular cluster systems. Mon. Not. R. Astron. Soc. 487, 1986–1993 (2019).

6. Wan, Z. et al. The tidal remnant of an unusually metal-poor globular cluster. Nature 583, 768–770 (2020).

7. Kruijssen, J. M. D. The minimum metallicity of globular clusters and its physical origin - implications for the galaxy mass-metallicity relation and observations of proto-globular clusters at high redshift. Mon. Not. R. Astron. Soc. 486, L20–L25 (2019).

8. Ibata, R. et al. Charting the Galactic acceleration field. I. A search for stellar streams with Gaia DR2 and EDR3 with follow-up from ESPaDOnS and UVES. Astrophys. J. 914, 123 (2021).

9. Gaia Collaboration et al. Gaia Early Data Release 3. Summary of the contents and survey properties. Astron. Astrophys. 649, A1 (2021).

10. Starkenburg, E. et al. The Pristine survey - I. Mining the Galaxy for the most metal-poor stars. Mon. Not. R. Astron. Soc. 471, 2587–2604 (2017).

11. Harris, W. E. A catalog of parameters for globular clusters in the Milky Way. Astron. J. 112, 1487 (1996).

12. Willman, B. & Strader, J. ”Galaxy,” defined. Astron. J. 144, 76 (2012).

13. Leaman, R. Insights into pre-enrichment of star clusters and self-enrichment of dwarf galaxies from their intrinsic metallicity dispersions. Astron. J. 144, 183 (2012).

14. Kirby, E. N. et al. The universal stellar mass-stellar metallicity relation for dwarf galaxies. Astrophys. J. 779, 102 (2013).

15. Gratton, R. G., Carretta, E. & Bragaglia, A. Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters. Astron. Astrophys. R. 20, 50 (2012).

16. Bastian, N. & Lardo, C. Multiple stellar populations in globular clusters. Ann. Rev. Astron. Astrophys. 56, 83–136 (2018).

17. Ji, A. P. et al. The Southern Stellar Stream Spectroscopic Survey (S5): chemical abundances of seven stellar streams. Astron. J. 160, 181 (2020).

18. Roederer, I. U. Are there any stars lacking neutron-capture elements? Evidence from strontium and barium. Astron. J. 145, 26 (2013).

19. Côté, B. et al. Neutron star mergers might not be the only source of r-process elements in the Milky Way. Astrophys. J. 875, 106 (2019).

20. Ji, A. P., Frebel, A., Simon, J. D. & Chiti, A. Complete element abundances of nine stars in the r-process galaxy Reticulum II. Astrophys. J. 830, 93 (2016).

21. Hansen, T. T. et al. An r-process enhanced star in the dwarf galaxy Tucana III. Astrophys. J. 838, 44 (2017).

22. Roederer, I. U. Primordial r-process dispersion in metal-poor globular clusters. Astrophys. J. Lett. 732, L17 (2011).

23. Yoon, J. et al. Galactic archeology with the AEGIS survey: the evolution of carbon and iron in the Galactic halo. Astrophys. J. 861, 146 (2018).

24. Norris, J. E. et al. The most metal-poor stars. IV. The two populations with [Fe/H] < −3.0. Astrophys. J. 762, 28 (2013).

25. Youakim, K. et al. The Pristine survey - VIII. The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime. Mon. Not. R. Astron. Soc. 492, 4986–5002 (2020).

26. Roederer, I. U. & Gnedin, O. Y. High-resolution optical spectroscopy of stars in the Sylgr stellar stream. Astrophys. J. 883, 84 (2019).

27. Larsen, S. S., Romanowsky, A. J., Brodie, J. P. & Wasserman, A. An extremely metal-deficient globular cluster in the Andromeda galaxy. Science 370, 970–973 (2020).

28. Bonaca, A., Hogg, D. W., Price-Whelan, A. M. & Conroy, C. The spur and the gap in GD-1: dynamical evidence for a dark substructure in the Milky Way halo. Astrophys. J. 880, 38 (2019).

29. Ibata, R. A., Lewis, G. F. & Martin, N. F. Feeling the pull: a study of natural galactic accelerometers. I. Photometry of the delicate stellar stream of the Palomar 5 globular cluster. Astrophys. J. 819, 1 (2016).

30. Erkal, D., Koposov, S. E. & Belokurov, V. A sharper view of Pal 5’s tails: discovery of stream perturbations with a novel non-parametric technique. Mon. Not. R. Astron. Soc. 470, 60–84 (2017).

31. Ma, X. et al. The origin and evolution of the galaxy mass-metallicity relation. Mon. Not. R. Astron. Soc. 456, 2140–2156 (2016).

32. Kielty, C. L. et al. The Pristine survey - XII. Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy. Mon. Not. R. Astron. Soc. 506, 1438–1461 (2021).

33. Malhan, K., Ibata, R. A. & Martin, N. F. Ghostly tributaries to the Milky Way: charting the halo’s stellar streams with the Gaia DR2 catalogue. Mon. Not. R. Astron. Soc. 481, 3442–3455 (2018).

34. Carlberg, R. G. Globular clusters in a cosmological N-body simulation. Astrophys. J. 861, 69 (2018).

35. Malhan, K., Ibata, R. A., Carlberg, R. G., Valluri, M. & Freese, K. Butterfly in a cocoon, understanding the origin and morphology of globular cluster streams: the case of GD-1. Astrophys. J. 881, 106 (2019).

36. Martin, N. F. et al. The Pristine survey – XVI. The metallicity of 21 stellar streams around the Milky Way detected with the STREAMFINDER in Gaia EDR3. Mon. Not. R. Astron. Soc submitted (2021).

37. Bressan, A. et al. PARSEC: stellar tracks and isochrones with the PAdova and TRieste stellar evolution code. Mon. Not. R. Astron. Soc. 427, 127–145 (2012).

38. Maraston, C. Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies. Mon. Not. R. Astron. Soc. 362, 799–825 (2005).

39. Deason, A. J., Belokurov, V. & Evans, N. W. The Milky Way stellar halo out to 40 kpc: squashed, broken but smooth. Mon. Not. R. Astron. Soc. 416, 2903–2915 (2011).

40. Lindegren, L. et al. Gaia Early Data Release 3. Parallax bias versus magnitude, colour, and position. Astron. Astrophys. 649, A4 (2021).

41. Ibata, R. A., Malhan, K., Martin, N. F. & Starkenburg, E. Phlegethon, a nearby 75-degree-long retrograde Stellar Stream. Astrophys. J. 865, 85 (2018).

42. Chene, A.-N. et al. in Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9151 (eds. Navarro, R. et al.) 915147 (SPIE, 2014).

43. Pazder, J., Fournier, P., Pawluczyk, R. & van Kooten, M. in Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 9151 (eds. Navarro, R. et al.) 915124 (SPIE, 2014).

44. Martioli, E. et al. in Software and Cyberinfrastructure for Astronomy II Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 8451 (eds. Radziwill, N. M. & Chiozzi, G.) 84512B (SPIE, 2012).

45. Schlegel, D. J., Finkbeiner, D. P. & Davis, M. Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525 (1998).

46. Schlafly, E. F. & Finkbeiner, D. P. Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD. Astrophys. J. 737, 103 (2011).

47. González Hernández, J. I. & Bonifacio, P. A new implementation of the infrared flux method using the 2MASS catalogue. Astron. Astrophys. 497, 497–509 (2009).

48. Mucciarelli, A., Bellazzini, M. & Massari, D. Exploiting the Gaia EDR3 photometry to derive stellar temperatures. Astron. Astrophys. (in the press).

49. Mashonkina, L., Jablonka, P., Pakhomov, Y., Sitnova, T. & North, P. The formation of the Milky Way halo and its dwarf satellites; a NLTE-1D abundance analysis. I. Homogeneous set of atmospheric parameters. Astron. Astrophys. 604, A129 (2017).

50. Karovicova, I. et al. Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars. Astron. Astrophys. 640, A25 (2020).

51. Giribaldi, R. E., da Silva, A. R., Smiljanic, R. & Cornejo Espinoza, D. TITANS metal-poor reference stars. I. Accurate effective temperatures and surface gravities for dwarfs and subgiants from 3D non-LTE H α profiles and Gaia parallaxes. Astron. Astrophys. 650, A194 (2021).

52. Kurucz, R. L. ATLAS12, SYNTHE, ATLAS9, WIDTH9, et cetera. Mem. Soc. Astron. Ital. Suppl. 8, 14 (2005).

53. Sneden, C. A. Carbon and Nitrogen Abundances in Metal-Poor Stars. PhD thesis, Univ. of Texas at Austin (1973).

54. Sobeck, J. S. et al. The abundances of neutron-capture species in the very metal-poor globular cluster M15: a uniform analysis of red giant branch and red horizontal branch stars. Astron. J. 141, 175 (2011).

55. Placco, V. M. et al. Linemake: an atomic and molecular line list generator. Res. Notes AAS 5, 92 (2021).

56. Lind, K., Bergemann, M. & Asplund, M. Non-LTE line formation of Fe in late-type stars - II. 1D spectroscopic stellar parameters. Mon. Not. R. Astron. Soc. 427, 50–60 (2012).

57. Bergemann, M. & Cescutti, G. Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy (2010).

58. Bergemann, M., Lind, K., Collet, R., Magic, Z. & Asplund, M. Non-LTE line formation of Fe in late-type stars - I. Standard stars with 1D and <3D> model atmospheres. Mon. Not. R. Astron. Soc. 427, 27–49 (2012).

59. Mashonkina, L., Korn, A. J. & Przybilla, N. A non-LTE study of neutral and singly-ionized calcium in late-type stars. Astron. Astrophys. 461, 261–275 (2007).

60. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Ann. Rev. Astron. Astrophys. 47, 481–522 (2009).

61. Tody, D. in Astronomical Data Analysis Software and Systems II Astronomical Society of the Pacific Conference Series, Vol. 52 (eds. Hanisch, R. J. et al.) 173 (1993).

62. Aguado, D. S., Allende Prieto, C., González Hernández, J. I., Rebolo, R. & Caffau, E. New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy. Astron. Astrophys. 604, A9 (2017).

63. Aguado, D. S., González Hernández, J. I., Allende Prieto, C. & Rebolo, R. J0815+4729: a chemically primitive dwarf star in the galactic halo observed with Gran Telescopio Canarias. Astrophys. J. Lett. 852, L20 (2018).

64. Allende Prieto, C. et al. Deep SDSS optical spectroscopy of distant halo stars. I. Atmospheric parameters and stellar metallicity distribution. Astron. Astrophys. 568, A7 (2014).

65. Aguado, D. S., González Hernández, J. I., Allende Prieto, C. & Rebolo, R. WHT follow-up observations of extremely metal-poor stars identified from SDSS and LAMOST. Astron. Astrophys. 605, A40 (2017).

66. Koesterke, L., Allende Prieto, C. & Lambert, D. L. Center-to-limb variation of solar three-dimensional hydrodynamical simulations. Astrophys. J. 680, 764–773 (2008).

67. Boender, C. G. E., Rinnoy Kan, A. H. G., Timmer, G. T. & Stougie, L. A stochastic method for global optimization. Math. Program. 22, 125 (1982).

68. Wenger, M. et al. The SIMBAD astronomical database. The CDS reference database for astronomical objects. Astron. Astrophys. Suppl. Ser. 143, 9–22 (2000).

69. Ochsenbein, F., Bauer, P. & Marcout, J. The VizieR database of astronomical catalogues. Astron. Astrophys. Suppl. Ser. 143, 23–32 (2000).

## Author information

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.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

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

## 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).

## Rights and permissions

Reprints and Permissions

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

• Accepted:

• Published:

• Issue Date:

• DOI: https://doi.org/10.1038/s41586-021-04162-2