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.
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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. (email@example.com). Source data are provided with this paper.
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.
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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.
The authors declare no competing interests.
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Extended data figures and tables
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.
a–e, 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 (b–d; 1σ uncertainty) show the observed values.
Extended Data Fig. 5 Comparison of energy E and actions Jϕ,R,z for the Phoenix stream and all Milky Way globular clusters.
a–c, 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.
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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