Skip to main content

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.

  • Article
  • Published:

The tidal remnant of an unusually metal-poor globular cluster

Abstract

Globular clusters are some of the oldest bound stellar structures observed in the Universe1. They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the hierarchical build-up of structure2,3. Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a metallicity floor, whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun4,5,6. The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form—both critical components for understanding the build-up of mass in the Universe7. Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance ([Fe/H] = −2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution.

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

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Metallicity versus spectroscopic signal-to-noise ratio for Phoenix stream members.
Fig. 2: Comparison of the summed equivalent widths of the Ca ii triplet.

Similar content being viewed by others

Data availability

The data used in this paper is from the S5 internal data release version 1.5; see https://s5collab.github.io. The first public data release is scheduled for the end of 2020, which will contain the observations taken in 2018 and 2019. Data requests and enquiries about the S5 collaboration should be directed to T.S.L. (tingli@carnegiescience.edu). Source data are provided with this paper.

Code availability

The 2DFDR for the raw data reduction is available at https://www.aao.gov.au/science/software/2dfdr. The RVSPECFIT32 used for the determination of stellar parameters is available at https://github.com/segasai/rvspecfit. Documents for the publication of the mixture model and the dynamical model code are under preparation. Results from the mixture model are available on request.

References

  1. Harris, W. E. Globular cluster systems in galaxies beyond the Local Group. Annu. Rev. Astron. Astrophys. 29, 543–579 (1991).

    ADS  Google Scholar 

  2. Brodie, J. P. & Strader, J. Extragalactic globular clusters and galaxy formation. Annu. Rev. Astron. Astrophys. 44, 193–267 (2006).

    ADS  CAS  Google Scholar 

  3. Mackey, D. et al. Two major accretion epochs in M31 from two distinct populations of globular clusters. Nature 574, 69–71 (2019).

    ADS  CAS  PubMed  Google Scholar 

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

    ADS  Google Scholar 

  5. Forbes, D. A. et al. Globular cluster formation and evolution in the context of cosmological galaxy assembly: open questions. Proc. R. Soc. Lond. A 474, 20170616 (2018).

    ADS  MathSciNet  MATH  Google Scholar 

  6. 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).

    ADS  CAS  Google Scholar 

  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).

    ADS  CAS  Google Scholar 

  8. Balbinot, E. et al. The Phoenix stream: a cold stream in the southern hemisphere. Astrophys. J. 820, 58 (2016).

    ADS  Google Scholar 

  9. Shipp, N. et al. Stellar streams discovered in the Dark Energy Survey. Astrophys. J. 862, 114 (2018).

    ADS  Google Scholar 

  10. Erkal, D., Sanders, J. L. & Belokurov, V. Stray, swing and scatter: angular momentum evolution of orbits and streams in aspherical potentials. Mon. Not. R. Astron. Soc. 461, 1590–1604 (2016).

    ADS  Google Scholar 

  11. Grillmair, C. J. & Carlberg, R. G. What a tangled web we weave: Hermus as the northern extension of the Phoenix stream. Astrophys. J. 820, L27 (2016).

    ADS  Google Scholar 

  12. Carlberg, R. G. & Grillmair, C. J. Velocity variations in the Phoenix-Hermus star stream. Astrophys. J. 830, 135 (2016).

    ADS  Google Scholar 

  13. Li, T. S. et al. The Southern Stellar Stream Spectroscopic Survey (S5): overview, target selection, data reduction, validation, and early science. Mon. Not. R. Astron. Soc. 490, 3508–3531 (2019).

    ADS  Google Scholar 

  14. Abbott, T. M. C. et al. The Dark Energy Survey: data release 1. Astrophys. J. Suppl. Ser. 239, 18 (2018).

    ADS  CAS  Google Scholar 

  15. Gaia Collaboration. The Gaia mission. Astron. Astrophys. 595, A1 (2016).

    Google Scholar 

  16. Gaia Collaboration. Gaia data release 2. Summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).

    Google Scholar 

  17. Usher, C. et al. The WAGGS project - II. The reliability of the calcium triplet as a metallicity indicator in integrated stellar light. Mon. Not. R. Astron. Soc. 482, 1275–1303 (2019).

    ADS  CAS  Google Scholar 

  18. Simon, J. D. The faintest dwarf galaxies. Annu. Rev. Astron. Astrophys. 57, 375–415 (2019).

    ADS  Google Scholar 

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

    ADS  Google Scholar 

  20. Simon, J. D. Gaia proper motions and orbits of the ultra-faint Milky Way satellites. Astrophys. J. 863, 89 (2018).

    ADS  Google Scholar 

  21. Gaia Collaboration. Gaia data release 2. Kinematics of globular clusters and dwarf galaxies around the Milky Way. Astron. Astrophys. 616, A12 (2018).

    Google Scholar 

  22. Vasiliev, E. Proper motions and dynamics of the Milky Way globular cluster system from Gaia DR2. Mon. Not. R. Astron. Soc. 484, 2832–2850 (2019).

    ADS  CAS  Google Scholar 

  23. Simpson, J. D. The most metal-poor Galactic globular cluster: the first spectroscopic observations of ESO280–SC06. Mon. Not. R. Astron. Soc. 477, 4565–4576 (2018).

    ADS  CAS  Google Scholar 

  24. Simpson, J. D. & Martell, S. L. A nitrogen-enhanced metal-poor star discovered in the globular cluster ESO280–SC06. Mon. Not. R. Astron. Soc. 490, 741–751 (2019).

    ADS  Google Scholar 

  25. Larsen, S. S., Brodie, J. P. & Strader, J. Detailed abundance analysis from integrated high-dispersion spectroscopy: globular clusters in the Fornax dwarf spheroidal. Astron. Astrophys. 546, A53 (2012).

    ADS  Google Scholar 

  26. Kruijssen, J. M. D. Globular clusters as the relics of regular star formation in ‘normal’ high-redshift galaxies. Mon. Not. R. Astron. Soc. 454, 1658–1686 (2015).

    ADS  CAS  Google Scholar 

  27. Iorio, G. & Belokurov, V. The shape of the Galactic halo with Gaia DR2 RR Lyrae. Anatomy of an ancient major merger. Mon. Not. R. Astron. Soc. 482, 3868–3879 (2019).

    ADS  CAS  Google Scholar 

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

    ADS  CAS  Google Scholar 

  29. Renzini, A. Finding forming globular clusters at high redshifts. Mon. Not. R. Astron. Soc. 469, L63–L67 (2017).

    ADS  CAS  Google Scholar 

  30. Simpson, Jeffrey D. Empirical relationship between calcium triplet equivalent widths and [Fe/H] using Gaia photometry (version 0.2) [data set]. Zenodo https://doi.org/10.5281/zenodo.3785756 (2020).

  31. AAO Software Team. 2dfdr: data reduction software, https://www.aao.gov.au/science/software/2dfdr (2015).

  32. Koposov, S. E. et al. Accurate stellar kinematics at faint magnitudes: application to the Boötes I dwarf spheroidal galaxy. Astrophys. J. 736, 146 (2011).

    ADS  Google Scholar 

  33. Husser, T. O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, A6 (2013).

    Google Scholar 

  34. Carrera, R., Pancino, E., Gallart, C. & del Pino, A. The near-infrared Ca II triplet as a metallicity indicator - II. Extension to extremely metal-poor metallicity regimes. Mon. Not. R. Astron. Soc. 434, 1681–1691 (2013).

    ADS  CAS  Google Scholar 

  35. Shipp, N. et al. Proper motions of stellar streams discovered in the Dark Energy Survey. Astrophys. J. 885, 3 (2019).

    ADS  CAS  Google Scholar 

  36. Schönrich, R., Binney, J. & Dehnen, W. Local kinematics and the local standard of rest. Mon. Not. R. Astron. Soc. 403, 1829–1833 (2010).

    ADS  Google Scholar 

  37. Bland-Hawthorn, J. & Gerhard, O. The galaxy in context: structural, kinematic, and integrated properties. Annu. Rev. Astron. Astrophys. 54, 529–596 (2016).

    ADS  CAS  Google Scholar 

  38. Li, T. S. et al. The first tidally disrupted ultra-faint dwarf galaxy? A spectroscopic analysis of the Tucana III stream. Astrophys. J. 866, 22 (2018).

    ADS  Google Scholar 

  39. Feroz, F. & Hobson, M. P. Multimodal nested sampling: an efficient and robust alternative to Markov chain Monte Carlo methods for astronomical data analyses. Mon. Not. R. Astron. Soc. 384, 449–463 (2008).

    ADS  Google Scholar 

  40. Feroz, F., Hobson, M. P. & Bridges, M. MULTINEST: an efficient and robust Bayesian inference tool for cosmology and particle physics. Mon. Not. R. Astron. Soc. 398, 1601–1614 (2009).

    ADS  Google Scholar 

  41. Marigo, P. et al. A new generation of PARSEC-COLIBRI stellar isochrones including the TP-AGB phase. Astrophys. J. 835, 77 (2017).

    ADS  Google Scholar 

  42. Erkal, D. et al. The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream. Mon. Not. R. Astron. Soc. 487, 2685–2700 (2019).

    ADS  CAS  Google Scholar 

  43. Gibbons, S. L. J., Belokurov, V. & Evans, N. W. ‘Skinny Milky Way please’, says Sagittarius. Mon. Not. R. Astron. Soc. 445, 3788–3802 (2014).

    ADS  CAS  Google Scholar 

  44. McMillan, P. J. The mass distribution and gravitational potential of the Milky Way. Mon. Not. R. Astron. Soc. 465, 76–94 (2017).

    ADS  CAS  Google Scholar 

  45. Dehnen, W. & Binney, J. Mass models of the Milky Way. Mon. Not. R. Astron. Soc. 294, 429–438 (1998).

    ADS  Google Scholar 

  46. Hernquist, L. An analytical model for spherical galaxies and bulges. Astrophys. J. 356, 359–364 (1990).

    ADS  Google Scholar 

  47. Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pacif. 125, 306–312 (2013).

    ADS  Google Scholar 

  48. Amorisco, N. C., Gómez, F. A., Vegetti, S. & White, S. D. M. Gaps in globular cluster streams: giant molecular clouds can cause them too. Mon. Not. R. Astron. Soc. 463, L17–L21 (2016).

    ADS  CAS  Google Scholar 

  49. 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).

    ADS  CAS  Google Scholar 

  50. Pearson, S., Price-Whelan, A. M. & Johnston, K. V. Gaps and length asymmetry in the stellar stream Palomar 5 as effects of Galactic bar rotation. Nat. Astron. 1, 633–639 (2017).

    ADS  Google Scholar 

  51. Banik, N. & Bovy, J. Effects of baryonic and dark matter substructure on the Pal 5 stream. Mon. Not. R. Astron. Soc. 484, 2009–2020 (2019).

    ADS  CAS  Google Scholar 

  52. Vasiliev, E. AGAMA: action-based galaxy modelling architecture. Mon. Not. R. Astron. Soc. 482, 1525–1544 (2019).

    ADS  Google Scholar 

  53. Price-Whelan, A. M. et al. Kinematics of the Palomar 5 stellar stream from RR Lyrae stars. Astron. J. 158, 223 (2019).

    ADS  CAS  Google Scholar 

  54. Koposov, S. E. et al. Piercing the Milky Way: an all-sky view of the Orphan stream. Mon. Not. R. Astron. Soc. 485, 4726–4742 (2019).

    ADS  CAS  Google Scholar 

  55. Foreman-Mackey, D. corner.py: scatterplot matrices in python. J. Open Source Softw. 24, https://doi.org/10.21105/joss.00024 (2016).

Download references

Acknowledgements

This work is part of the ongoing S5 (https://s5collab.github.io). The work is based in part on data acquired through the Australian Astronomical Observatory, under program A/2018B/09. We acknowledge the traditional owners of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past, present and emerging. We thank P. McMillan for providing the posterior chains for his fit to the Milky Way potential44. This project used public archival data from the DES. Funding for DES projects has been provided by the DOE and NSF (USA), MISE (Spain), STFC (UK), HEFCE (UK), NCSA (UIUC), KICP (U. Chicago), CCAPP (Ohio State), MIFPA (Texas A&M), CNPQ, FAPERJ, FINEP (Brazil), MINECO (Spain), DFG (Germany) and the collaborating institutions in the DES, which are Argonne Lab, UC Santa Cruz, University of Cambridge, CIEMAT-Madrid, University of Chicago, University College London, DES-Brazil Consortium, University of Edinburgh, ETH Zürich, Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE Barcelona, Lawrence Berkeley Lab, LMU München and the associated Excellence Cluster Universe, University of Michigan, NOAO, University of Nottingham, Ohio State University, OzDES Membership Consortium, University of Pennsylvania, University of Portsmouth, SLAC National Lab, Stanford University, University of Sussex, and Texas A&M University. This work is based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This work has made use of data from the European Space Agency (ESA) 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 DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Parts of this research were conducted by the Australian Research Council (ARC) Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. Z.W. is supported by a Dean’s International Postgraduate Research Scholarship at the University of Sydney. D.M. is supported by an ARC Future Fellowship (FT160100206). J.D.S., S.L.M. and D.B.Z. acknowledge the support of the ARC through Discovery Project grant DP180101791. T.S.L. and A.P.J. are supported by NASA through Hubble Fellowship grants HST-HF2-51439.001 and HST-HF2-51393.001, respectively, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy for NASA, under contract NAS5-26555.

Author information

Authors and Affiliations

Authors

Contributions

The S5 programme was initiated by T.S.L., D.B.Z., K.K. and G.F.L. Survey design and target selection for S5 was undertaken by T.S.L. and N.S. Observations with the AAT were performed by G.F.L., K.K., D.M., S.L.M., J.D.S., D.B.Z., G.S.D.C. and Z.W. Data reduction, calibration and analysis was undertaken by S.E.K., T.S.L., A.P.J., Z.W. and G.F.L. D.E. performed the dynamical analysis, including stream fitting, orbit determination and action comparison. All authors were involved in the discussion and interpretation of the results presented, and all contributed to writing the paper.

Corresponding author

Correspondence to Geraint F. Lewis.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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 The observational properties of the Phoenix member stars.

a, The on-sky distribution of all stars observed in the 2dF fields targeting the Phoenix stream. The overall footprint is a series of circular 2dF pointings. R.A., right ascension; Dec., declination. b, Radial velocity in the Galactic standard of rest (RVGSR) versus stream longitude (ϕ1) for Phoenix stars selected on the basis of proper motion, photometry and the mixture model. On the basis of the approximately linear correlation between RVGSR and ϕ1, we select Phoenix stream members from the region between the dashed lines ((1.02ϕ1 – 60.7) km s−1 < RVGSR < (1.02ϕ1 – 42.7) km s−1), which effectively excludes non-members (shown as small pink circles). c, The colour–magnitude diagram of stars selected as members of the Phoenix stream. Over-plotted are PADOVA isochrones41 with [Fe/H] = −2.9 to [Fe/H] = −2.0 (from blue to red), m − M = 16.4 (ref. 9, where m − M is the distance modulus, m is the apparent magnitude and M is the absolute magnitude) and log10[age (Gyr)] = 10.05. In all panels, the stars we identify as members of the Phoenix stream are represented by larger circles; those with high signal-to-noise ratio are colour-coded by their metallicity, others are grey. The four orange squares indicate the BHB and RR Lyrae stars, metallicities of which cannot be measured using the method used here.

Source data

Extended Data Fig. 2 The posterior sampling results of the metallicity distribution of the 11 Phoenix member stars with signal-to-noise ratios greater than 10.

The mean and dispersion of the metallicity are noted. The dispersion is consistent with being zero, with σ[Fe/H] < 0.2 being the 95% confidence interval. This figure is made using the corner package55.

Source data

Extended Data Fig. 3 The posterior sampling results of the RVGSR distribution.

The parameters p0, p1 and p2 are the best-fitting polynomial parameters for RVGSR(ϕ1) = p0 + p1ϕ1 + p2ϕ12; σrv is the intrinsic dispersion. Here the best-fitting parameters are calculated with ϕ1 in radians. This figure is made using the corner package55.

Source data

Extended Data Fig. 4 Best-fit model to the Phoenix stream.

ae, The stream on the sky (a), the proper motions in right ascension (μα*; b) and declination (μδ; c), the residuals of the radial velocity (Δvr; d) and the distance to the stream (r; e). The blue points show the best-fit model and the red points (a) or error bars (bd; 1σ uncertainty) show the observed values.

Source data

Extended Data Fig. 5 Comparison of energy E and actions Jϕ,R,z for the Phoenix stream and all Milky Way globular clusters.

ac, The actions are computed with AGAMA52 in the best-fit Milky Way potential44. Pal 5 (red circles) is the closest in energy and actions to the Phoenix stream (green star), suggesting a possible association. There is also a potential relation in this space to NGC 5053 (blue circles), another globular cluster. All other global clusters are shown in black.

Source data

Source data

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Wan, Z., Lewis, G.F., Li, T.S. et al. The tidal remnant of an unusually metal-poor globular cluster. Nature 583, 768–770 (2020). https://doi.org/10.1038/s41586-020-2483-6

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-020-2483-6

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing