The Large Magellanic Cloud isn’t Very Metal

The Large Magellanic Cloud (LMC) is the Milky Way’s most massive satellite galaxy. Because it’s so easily observed, astronomers have studied it intently. They’re interested in how star formation in the LMC might have been different than in the Milky Way.

A team of researchers zeroed in on the LMC’s most metal-deficient stars to find out how different.

The LMC is about 163,000 light-years away and about 32,000 light-years across. Even though it’s that large, it’s still only 1/100th the mass of the Milky Way. It was probably a dwarf spiral galaxy before gravitational interactions with the Milky Way and the Small Magellanic Cloud warped its shape. Scientists predict it’ll probably merge with the Milky Way in about 2.4 billion years.

The LMC wasn’t always this close to the Milky Way. It formed elsewhere in the Universe, out of a different reservoir of gas than the Milky Way. The LMC’s stars preserve the environmental conditions they formed in.

The first stars to form in the Universe were the most metal-poor stars. When they formed, only hydrogen and helium from the Big Bang were available. These stars are called Population 3 stars, and they’re largely hypothetical. They were massive and many of them exploded as supernovae. These stars forged the heavier elements, called metals in astronomy, and then spread them out into space to be taken up by the next stars to form. That process continued generation by generation.

Population III stars were the Universe’s first stars. They were extremely massive, luminous stars, and many exploded as supernovae. Image Credit: DALL-E

Nobody’s ever found a Population 3 star because even if they’re more than hypothetical, they’d all be long gone by now. But in new research, scientists examined 10 of the LMC’s most metal-poor stars. They found one Population 2 star that is so metal-poor it’s similar to Population 3 stars.

The research is titled “Enrichment by extragalactic first stars in the Large Magellanic Cloud.” It’s published in the journal Nature Astronomy. The lead author is Anirudh Chiti from the Department of Astronomy & Astrophysics and the Kavli Institute for Cosmological Physics, both at the University of Chicago.

“This star provides a unique window into the very early element-forming process in galaxies other than our own,” said lead author Chiti. “We have built up an idea of how these stars that were chemically enriched by the first stars look like in the Milky Way, but we don’t yet know if some of these signatures are unique or if things happened similarly across other galaxies.”

The earliest Population 3 stars changed the Universe. By producing metals, they guaranteed the stars to follow had higher metallicities. But exactly what metals did they produce, and how much?

“We want to understand what the properties of those first stars were and what were the elements they produced,” said Chiti.

The difficult part is that nobody’s ever seen a Population 3 star. But by identifying an extremely metal-poor star that’s very similar to the first stars, the researchers found the next best thing. Finding nine other metal-poor stars was also helpful.

They compared the 10 LMC metal-poor stars to metal-poor stars in the Milky Way. The results show how different processes and different environments in both galaxies affected star formation and metal enrichment.

This illustration shows the Milky Way galaxy’s inner and outer halos. Old, metal-poor stars tend to inhabit the halo. (Image Credits: NASA, ESA, and A. Feild [STScI])

These metal-poor stars are difficult to find. Most of the stars in the Universe resulted from successive generations of stars; their enriched metallicity is a testament to that. Our Sun is a metal-rich Population 1 star, for example.

But these older, metal-poor Population 2 stars are out there. Since astronomers will likely never find an ancient Population 3 star, the Population 2 stars with the lowest metallicities are the next best things.

“Maybe fewer than 1 in 100,000 stars in the Milky Way is one of these second-gen stars,” Chiti said. “You really are fishing needles out of haystacks.”

But once astronomers find them, the outer layers of these rare stars hold evidence of the conditions they formed in. “In their outer layers, these stars preserve the elements near where they formed,” Chiti explained. “If you can find a very old star and get its chemical composition, you can understand what the chemical composition of the universe was like where that star formed billions of years ago.”

This figure from the study shows the ten LMC stars (blue crosses) compared to all stars within 10° of the LMC. They’re colour-coded with the Fe/H bar on the right. The Fe/H ratio shows the ratio of iron atoms to hydrogen atoms and is a common measure of overall metallicity. The scale on the left shows Calcium, Hydrogen, and Potassium abundances across the whole sky, another useful measure of metallicity. Image Credit: Chiti et al. 2024.

Finding such metal-poor stars in the LMC allowed astronomers to compare the star-forming conditions in that satellite galaxy to those in the Milky Way. The comparison can help astrophysicists understand how these star-forming conditions may have differed.

One of the 10 stars in the LMC stood out from the rest. It had markedly lower metallicity than the other nine. Called LMC 119, it’s 50 times more metal-deficient than the others. “Given its extremely low metallicity, this star exhibits the characteristics of a second-generation star that preserves the chemical imprints of a first-star supernova,” the authors write.

This figure from the research compares the atomic abundances of LM 119 to red giant stars in the Milky Way’s halo, where older, metal-poor stars are situated. As the figure shows, LMC 119 has much lower metallicity than the Milky Way’s metal-poor stars. Image Credit: Chiti et al. 2024.

One fact stood out to the researchers when they mapped LMC 119’s elements. It had much less carbon than iron when compared to Milky Way stars. In fact, the same was true of all 10 stars in the sample. This is important because the LMC wasn’t always a satellite galaxy of the Milky Way. That association only goes back a couple of billion years or so. Its stars formed in a distant region of the high-redshift Universe.

“That was very intriguing, and it suggests that perhaps carbon enhancement of the earliest generation, as we see in the Milky Way, was not universal,” Chiti said. “We’ll have to do further studies, but it suggests there are differences from place to place.”

For Chiti and his colleagues, the conclusion is clear. “This, and other abundance differences, affirm that the extragalactic early LMC experienced diverging enrichment processes compared to the early Milky Way. Early element production, driven by the earliest stars, thus appears to proceed in an environment-dependent manner,” they write in their conclusion.

The Large and Small Magellanic Clouds are visible at the lower right-hand corner of this image of the Milky Way as seen by the European Space Agency’s Gaia satellite. Image Credit: ESA/Gaia/DPAC

Since Chiti and his fellow researchers found one very low-metallicity star in the LMC, there are probably many more among its suspected population of 20 billion stars. Chiti is leading a program to map out more stars in the southern sky and find more of these types of stars.

“This discovery suggests there should be many of these stars in the Large Magellanic Cloud if we look closely,” he said. “It’s really exciting to be opening up stellar archeology of the Large Magellanic Cloud and to be able to map out in such detail how the first stars chemically enriched the universe in different regions.”

Evan Gough

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