Ancient galaxy study forces astrophysicists to rethink cosmic origins of fluorine - the element in your bones and teeth

Fluorine has long been linked to Wolf-Rayet stars, very massive stellar objects that burn fast, blow away their outer layers, and die quickly.

The idea gained even more traction in 2021 when scientists reported detecting fluorine in a galaxy about 12.4 billion light-years away, when the Universe was just 1.4 billion years old.

But the new research by Chiaki Kobayashi, Professor of Astrophysics at the University of Hertfordshire’s Centre for Astrophysics Research, and published in The Astrophysical Journal Letters, challenges the prevailing view that Wolf-Rayet stars seeded the early Universe with large amounts of fluorine.

An international group of researchers, which also involved scientists in Japan, analysed an even earlier galaxy, known as G09.83808, dating back roughly 12.9 billion years, where stars were forming very rapidly.

The researchers searched for hydrogen fluoride in the galaxy’s spectrum - a molecule used as a tracer for fluorine - but found no clear signal.

Models suggested that if Wolf-Rayet stars were the main producers of fluorine, the signal should have been detectable.

Because this was not the case, the researchers concluded these stars were unlikely to be the key source of fluorine in that galaxy.

Prof Kobayashi said: “Wolf-Rayet stars are being closely studied as a potential source of nitrogen in very-early galaxies recently discovered by the James Webb Space Telescope (JWST) - the most powerful infrared space observatory ever built. These stars have been the leading candidate also for fluorine production in the early Universe, but our observations suggest the picture is far more complex than we thought.

“We need to study more galaxies and refine our models to understand the cosmic origins of this key element.”

Understanding how fluorine formed in the early Universe could help explain the origin of all other elements, from which the Earth, human body, and our environment is made of.

Astronomers believe fluorine in the Universe comes from three main sources: Wolf-Rayet stars; Asymptotic Giant Branch (AGB) stars - elderly, bloated stars shedding their outer layers in the final stages of normal stars like our Sun; and core-collapse supernovae, the explosive deaths of massive stars.

Most fluorine in the Universe today is thought to come from AGB stars. But because these only appear at the end of long-lived stars, AGB stars cannot explain how fluorine came to exist in the early Universe - when there simply hadn't been enough time for them to evolve.

That led scientists to focus on short-lived, Wolf-Rayet stars instead. They were seen as the key candidates capable of producing fluorine so early in cosmic history.

In 2021, an international team including Prof Kobayashi and colleagues at Hertfordshire analysed an ancient galaxy, known as NGP-190387, and found levels of fluorine that suggested Wolf-Rayet stars may indeed have played a major role.

But there was a catch. Without precise information about the galaxy's physical conditions, the team could not confirm Wolf-Rayet stars were responsible.

To put the theory to a more rigorous test, researchers turned to a different ancient galaxy - G09.83808 - observed using the Atacama Large Millimeter/submillimeter Array, one of the world's most powerful radio observatories in the Chilean desert.

This galaxy was chosen because its physical properties are unusually well understood, and because it is magnified by gravitational lensing - a phenomenon where light bends around massive objects in space, making the galaxy appear brighter and allowing detailed observations in far less telescope time than would otherwise be needed.

Researchers say further observations of ancient galaxies will now be needed to determine which cosmic processes first forged fluorine.

Prof Kobayashi said: “The next step is to observe more ancient galaxies to increase the sample size and build a clearer picture of which cosmic processes were responsible.

“Each new observation brings us closer to understanding not just where fluorine came from - but how the Universe itself was chemically enriched.”

Learn more about our Centre for Astrophysics (CAR) research.