An extremely metal-deficient globular cluster in the Andromeda Galaxy

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Science  20 Nov 2020:
Vol. 370, Issue 6519, pp. 970-973
DOI: 10.1126/science.abb1970

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A relic star cluster under the floor

Globular clusters (GCs) are gravitationally bound assemblies of thousands to millions of stars that orbit in the outskirts of large galaxies. GCs consist of old stars with low metallicity containing low proportions of chemical elements heavier than hydrogen and helium. However, GCs appear to have a minimum metallicity, known as the floor, implying that at least some of those elements were required for their formation. Larsen et al. have found a GC in the nearby Andromeda Galaxy with a metallicity beneath the floor. This unexpected discovery will inform models of GC formation and incorporation into galaxies.

Science, this issue p. 970


Globular clusters (GCs) are dense, gravitationally bound systems of thousands to millions of stars. They are preferentially associated with the oldest components of galaxies, so measurements of their composition can constrain the build-up of chemical elements in galaxies during the early Universe. We report a massive GC in the Andromeda Galaxy (M31), RBC EXT8, that is extremely depleted in heavy elements. Its iron abundance is about 1/800 that of the Sun and about one-third that of the most iron-poor GCs previously known. It is also strongly depleted in magnesium. These measurements challenge the notion of a metallicity floor for GCs and theoretical expectations that massive GCs could not have formed at such low metallicities.

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