Low-metallicity globular star cluster challenges formation models

October 15, 2020

On the outskirts of the nearby Andromeda Galaxy, researchers have unexpectedly discovered a globular cluster (GC) - a massive congregation of relic stars - with a very low abundance of chemical elements heavier than hydrogen and helium (known as its metallicity), according to a new study. The GC, designated RBC EXT8, has 800 times lower abundance of these elements than the Sun, below a previously-observed limit, challenging the notion that massive GCs could not have formed at such low metallicities. GCs are dense, gravitationally bound collections of thousands to millions of ancient stars that orbit in the fringes of large galaxies; many GCs formed early in the history of the Universe. Because they contain some of the oldest stars in a galaxy, GCs provide astronomers with a record of early galaxy formation and evolution. The most metal-poor GCs have abundances about 300 times lower than the Sun and no GCs with metallicities below that value were previously known. This was thought to indicate a limit to metal content - a metallicity floor - that was required for GC formation; several mechanisms have been proposed to explain this limit. Søren Larsen and colleagues report the discovery of an extremely metal-deficient GC in the Andromeda Galaxy. Spectral analysis of RBC EXT8 shows that its metallicity is nearly three times lower than the most metal-poor clusters previously known, challenging the need for a metallicity floor. "Our finding shows that massive globular clusters could form in the early Universe out of gas that had only received a small 'sprinkling' of elements other than hydrogen and helium. This is surprising because this kind of pristine gas was thought to be associated with proto-galactic building blocks too small to form such massive star clusters," said Larsen.
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American Association for the Advancement of Science

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