The tidal remnant of an unusually metal-poor globular cluster

Abstract

Globular clusters are some of the oldest bound stellar structures observed in the Universe. They are ubiquitous in large galaxies and are believed to trace intense star formation events and the hierarchical build-up of structure. 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 Sun. 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 universe. 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 that of the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular cluster 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.

Authors' version of an Article published in Nature on July 29th, 2020

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• The Southern Stellar Stream Spectroscopic Survey (S5): Chemical Abundances of Seven Stellar Streams
• Orbital Clustering Identifies the Origins of Galactic Stellar Streams
• A stellar stream remnant of a globular cluster below the metallicity floor
• The Complexity of the Cetus Stream Unveiled from the Fusion of STREAMFINDER and StarGO
• VVV CL001: Likely the Most Metal-Poor Surviving Globular Cluster in the Inner Galaxy
• The Pristine survey -- XVII. The C-19 stream is dynamically hot and more extended than previously thought
• Simulations of globular clusters within their parent galaxies: Metallicity spreads and anomalous precursor populations
• APOGEE-2 Discovery of a Large Population of Relatively High-Metallicity Globular Cluster Debris
• Central X-ray point-sources found to be abundant in low-mass, late-type galaxies predicted to contain an intermediate-mass black hole
• The Phoenix stellar stream rose from the ashes of an ancient star cluster