Astronomers using the High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescope at W. M. Keck Observatory and the Canada-France-Hawaii Telescope (CFHT) have discovered a massive globular cluster in the Andromeda Galaxy, RBC EXT8, that is extremely poor in heavy elements — chemical elements heavier than hydrogen and helium. The cluster’s iron abundance is about 800 times lower than that of the Sun, and about three times lower than the most iron-poor globular clusters previously known. The cluster is also extremely deficient in magnesium.
The globular cluster RBC EXT8. Image credit: ESASky / CFHT.
Globular clusters 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.
Until now, astronomers thought large globular clusters had to contain a considerable amount of heavy elements.
Hydrogen and helium are the two main elements created after the Big Bang. Heavier elements such as iron and magnesium formed later.
Finding a massive globular cluster like RBC EXT8 that is extremely poor in metals defies current formation models, calling into question some of our ideas about the birth of stars and galaxies in the young Universe.
“I’m amazed that this remarkable star cluster was just sitting under our noses,” said Professor Aaron Romanowsky, an astronomer in the University of California Observatories and the Physics and Astronomy Department at San José State University.
“It is one of the brightest clusters in the Andromeda Galaxy and known for decades, yet no one had checked it out in detail.”
“It shows how the Universe still has many surprises for us to discover. It also reminds us to check our assumptions — in this case, it was assumed enough clusters had been investigated to know how anemic they can be.”
RBC EXT8 orbits the outskirts of the Andromeda Galaxy, which is located 2.5 million light-years from Earth. Image credit: ESASky / CFHT.
The astronomers made spectroscopic observations with the HIRES instrument to determine RBC EXT8’s metal content and used three archive images from CFHT to determine its size and estimate its mass.
“Our finding shows that massive globular clusters could form in the early Universe out of gas with only a small ‘sprinkling’ of elements other than hydrogen and helium,” said Dr. Søren Larsen, an astronomer in the Department of Astrophysics in the Institute of Mathematics, Astrophysics and Particle Physics at Radboud University.
“This is surprising because such pristine gas was thought to be in building blocks too small to form such massive star clusters.”
“This discovery is exciting because the idea of a ‘metallicity floor’ for globular clusters, that must contain some minimum amount of heavy metals, underpinned so much of our thinking about how these very old star clusters formed in the early Universe,” said Professor Jean Brodie, an astronomer in the University of California Observatories and director of the Centre for Astrophysics and Supercomputing at Swinburne University of Technology.
“Our finding contradicts the standard picture and that is always fun.”
The discovery is reported in the journal Science.
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Søren S. Larsen et al. An extremely metal-deficient globular cluster in the Andromeda Galaxy. Science, published online October 15, 2020; doi: 10.1126/science.abb1970
