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. The Phoenix stream in the Milky Way halo is shown to be a tidally disrupted remnant of an unusually metal-poor globular cluster, which was possibly destroyed during Galactic evolution.

中文翻译：

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一个异常贫金属球状星团的潮汐残余

球状星团是宇宙中观察到的最古老的束缚恒星结构之一。它们在大型星系中无处不在，据信可以追踪强烈的恒星形成事件和结构的层次结构2,3。对银河系中的球状星团和各种其他星系的观测发现了“金属丰度底”的证据，即没有发现化学（金属）丰度低于约 0.3% 至 0.4% 的球状星团。太阳 4-6。这种金属丰度底的存在可能反映了幸存的球状星团形成的最小质量和最大红移——这两个都是了解宇宙中质量积累的关键组成部分。在这里，我们报告了南方恒星流光谱调查对空间薄的测量结果，银河系光晕中动态冷凤凰星流。凤凰流的特性与它是一个球状星团的潮汐破坏残骸是一致的。然而，其金属丰度 ([Fe/H] = -2.7) 大大低于经验金属丰度底线。因此，凤凰流代表了迄今为止发现的最缺乏金属的球状星团的碎片，它的祖先与当地宇宙中当今的球状星团群不同。它的存在意味着金属丰度底面以下的球状星团可能已经存在，但在银河演化过程中被摧毁了。银河系晕中的凤凰流被证明是一个异常贫金属的球状星团的潮汐破坏残余物，它可能在银河演化过程中被摧毁。凤凰流的特性与它是一个球状星团的潮汐破坏残骸是一致的。然而，其金属丰度 ([Fe/H] = -2.7) 大大低于经验金属丰度底线。因此，凤凰流代表了迄今为止发现的最缺乏金属的球状星团的碎片，它的祖先与当地宇宙中当今的球状星团群不同。它的存在意味着金属丰度底面以下的球状星团可能已经存在，但在银河演化过程中被摧毁了。银河系晕中的凤凰流被证明是一个异常贫金属的球状星团的潮汐破坏残余物，它可能在银河演化过程中被摧毁。凤凰流的特性与它是一个球状星团的潮汐破坏残骸是一致的。然而，其金属丰度 ([Fe/H] = -2.7) 大大低于经验金属丰度底线。因此，凤凰流代表了迄今为止发现的最缺乏金属的球状星团的碎片，它的祖先与当地宇宙中当今的球状星团群不同。它的存在意味着金属丰度底面以下的球状星团可能已经存在，但在银河演化过程中被摧毁了。银河系晕中的凤凰流被证明是一个异常贫金属的球状星团的潮汐破坏残余物，它可能在银河演化过程中被摧毁。