The tidal remnant of an unusually metal-poor globular cluster

Zhen Wan, Geraint F. Lewis (), Ting S. Li, Jeffrey D. Simpson, Sarah L. Martell, Daniel B. Zucker, Jeremy R. Mould, Denis Erkal, Andrew B. Pace, Dougal Mackey, Alexander P. Ji, Sergey E. Koposov, Kyler Kuehn, Nora Shipp, Eduardo Balbinot, Joss Bland-Hawthorn, Andrew R. Casey, Gary S. Costa, Prajwal Kafle, Sanjib Sharma and Gayandhi M. Silva
Additional contact information
Zhen Wan: The University of Sydney
Geraint F. Lewis: The University of Sydney
Ting S. Li: Observatories of the Carnegie Institution for Science
Jeffrey D. Simpson: University of New South Wales
Sarah L. Martell: University of New South Wales
Daniel B. Zucker: Macquarie University
Jeremy R. Mould: Swinburne University of Technology
Denis Erkal: University of Surrey
Andrew B. Pace: Carnegie Mellon University
Dougal Mackey: Australian National University
Alexander P. Ji: Observatories of the Carnegie Institution for Science
Sergey E. Koposov: Carnegie Mellon University
Kyler Kuehn: Lowell Observatory
Nora Shipp: Fermi National Accelerator Laboratory
Eduardo Balbinot: Kapteyn Astronomical Institute, University of Groningen
Joss Bland-Hawthorn: The University of Sydney
Andrew R. Casey: Monash University
Gary S. Costa: Australian National University
Prajwal Kafle: The University of Western Australia
Sanjib Sharma: The University of Sydney
Gayandhi M. Silva: Centre of Excellence for All-Sky Astrophysics in Three Dimensions (ASTRO 3D)

Nature, 2020, vol. 583, issue 7818, 768-770

Abstract: 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 Sun4–6. 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.

Date: 2020
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DOI: 10.1038/s41586-020-2483-6

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