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A spherical star cluster has surprisingly few heavy elements

A strange group of newly measured stars orbiting the nearby Andromeda galaxy has the lowest level of heavy chemical elements ever seen in one of these mysterious star clusters. Called RBC EXT8, this globular cluster is also surprisingly massive and challenges theories of how those clusters and some galaxies form, astronomers reported online on Oct. 15 in Science.

“It’s a very unusual object,” says astrophysicist Oleg Gnedin of the University of Michigan at Ann Arbor, who was not involved in the new discovery.

Globular clusters are multitudinous spherical collections of stars that orbit around the center of a galaxy, although most, including RBC EXT8, live in galactic surroundings. Clusters are usually billions of years old, so their stars are usually chemically virgin, meaning they formed before the universe had time to create much of the heavier elements than hydrogen or helium, which astronomers put together as "metals."

Previous observations of these clusters in the Milky Way and other galaxies had suggested that there is a low limit that may be the metal content of a globular cluster. The poorest groups in metals were about 300 times less rich in heavy elements such as iron than the sun, but no less.

But the spectra of RBC EXT8, about 2.5 million light-years away, show that the metal content of the cluster is about 800 times lower than that of the sun. The globular cluster that had the previous record of lowest “metallicity” has three times that amount.

“It was completely unexpected that we found such a metal-poor globular cluster,” says astronomer Søren Larsen of Radboud University in Nijmegen, the Netherlands.

The largest and most diffuse spot in the image on the left is RBC EXT8, a globular cluster that orbits about 88,000 light-years from the center of the Andromeda Galaxy (shown on the right). A new study has found that the cluster has surprisingly few heavy elements for its size.© 2020 ESASky, CFHT

Moreover, given its metal-poor state, this cluster is surprisingly massive and weighs approximately 1.14 million times the mass of the sun. (An average-weight globular cluster is about 100,000 solar masses, but some clusters reach 3 million solar masses. The RBC EXT8 is heavy, but not the heaviest).

That mass makes the cluster even harder to explain because through the cosmos, the larger a galaxy or cluster, the more heavy elements it normally has.

There are several potential explanations for this trend, but one is simply that more massive galaxies or globular clusters have more stars. A star melts heavy elements in its core and sprinkles it around its host cluster or galaxy as it ages. Sufficiently massive stars can explode in a supernova, spreading those metals to be part of the next generation of stars (SN: 9/8/19). So more stars means more opportunity for metals to accumulate locally.

More massive objects also have the advantage of gravity, which allows them to better hold on to the metals they have and remain a cohesive group for billions of years. Less massive globular clusters dissolve in your host galaxy over time.

Those trends together could explain the apparent “metallicity floor” for globular clusters; all less massive and metal-poorer clusters have separated over the eons.

RBC EXT8 puts that conventional wisdom in your head. “It’s too big to have as low a metallicity as it has,” Gnedin says. "That's the riddle."

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Astronomers are unsure of how globular clusters form in general, but they probably grow inside galaxies, rather than forming outside of them and being attracted to them later. And so clusters reflect the characteristics of their galaxies: small metal-poor galaxies end up with small metal-poor globular clusters and vice versa. But based on the metal content of RBC EXT8, its galactic birthplace would be less than a million solar masses, as small as the globular cluster itself, which is a paradox.

As a result, the cluster challenges some simplified models of galaxy formation. But it doesn’t break them completely, Gnedin says. “It’s an object, it’s not going to knock things down,” he says. “It just makes people who work on these issues have to work harder” and have a more open mindset about other ways that galaxies could form.

The open-mindedness and willingness to explore are perhaps responsible for the new discovery about RBC EXT8 metals. Larsen and his colleagues saw the globular cluster at the start of an observation night with the Keck Telescope in Hawaii in October 2019. “It was really a serendipitous discovery,” he says. He had a free hour before the globular clusters of the M33 galaxy that his team was planning to watch rose above the horizon, so observers chose another cluster “more or less random” to fill time.

“At first, I really couldn’t believe what came out (in the observations) was right,” Larsen says. "But I kept working on it and it turned out to hold."

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