Astronomers make most distant detection yet of fluorine in star-forming galaxy

4 November 2021

A new discovery is shedding light on how fluorine — an element found in our bones and teeth as fluoride — is forged in the Universe.

Led by scientists at the University of Hertfordshire, the new study is the first time the element has been spotted in such a distant galaxy.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, partnered with the European Southern Observatory (ESO), a team of astronomers led by the University of Hertfordshire has detected fluorine in a galaxy that is so far away, its light has taken over 12 billion years to reach us.

This is the first time that the element has been spotted in such a distant star-forming galaxy.

“We all know about fluorine because the toothpaste we use every day contains it, in the form of fluoride,” says Maximilien Franco, Post-Doctoral Research Fellow at the University of Hertfordshire, who led the new study, published today in Nature Astronomy. Like most elements around us, fluorine is created inside stars; but until now we did not know exactly how this element was produced. “We did not even know which type of stars produced the majority of fluorine in the Universe”, explained Franco.

Franco and his collaborators – including Kristen Coppin, James Geach and Chiaki Kobayashi from the University of Hertfordshire – spotted fluorine (in the form of hydrogen fluoride) in the large clouds of gas of the distant galaxy NGP–190387, which we see as it was when the Universe was only 1.4 billion years old – about 10% of its current age. Since stars expel the elements they form in their cores as they reach the end of their lives, this detection implies that the stars that created fluorine must have lived and died quickly.

The team believes that Wolf–Rayet stars are the most likely production sites of fluorine. These are massive stars that live only a few million years – a blink of the eye in the Universe’s history. Wolf-Rayet stars had been suggested as possible sources of cosmic fluorine before, but astronomers did not know until now how important they were in producing this element in the early Universe.

“We have shown that Wolf–Rayet stars, which are among the most massive stars known and can explode violently as they reach the end of their lives, help us, in a way, to maintain good dental health!” jokes Franco.

Besides these stars, other scenarios for how fluorine is produced and expelled have been put forward in the past. An example includes pulsations of giant, evolved stars with masses up to a few times that of our Sun, called asymptotic giant branch stars. But the team believes these scenarios, some of which take billions of years to occur, might not fully explain the amount of fluorine in NGP–190387.

“For this galaxy, it took just tens or hundreds of millions of years to have fluorine levels comparable to those found in stars in the Milky Way, which is 13.5 billion years old. This was a totally unexpected result,” says Professor Chiaki Kobayashi, University of Hertfordshire. “Our measurement adds a completely new constraint on the origin of fluorine, which has been studied for two decades”.

The discovery in NGP–190387 marks one of the first detections of fluorine beyond the Milky Way and its neighbouring galaxies. Astronomers have previously spotted this element in distant quasars, bright objects powered by supermassive black holes at the centre of some galaxies. But never before had this element been observed in a star-forming galaxy so early in the history of the universe.

The team’s detection of fluorine was a chance discovery made possible thanks to the use of space and ground-based observatories. NGP–190387, originally discovered with the European Space Agency’s Herschel Space Observatory and later observed with the Chile-based ALMA, is extraordinarily bright for its distance. The ALMA data confirmed that the exceptional luminosity of NGP–190387 was partly caused by another known massive galaxy, located between NGP–190387 and the Earth, very close to the line of sight. This massive galaxy amplified the light observed by Franco and his collaborators, enabling them to spot the faint radiation emitted billions of years ago by the fluorine in NGP–190387.

This research was presented in the paper "Hydrogen fluoride at z > 4: witnessing the ramp-up of enrichment of the interstellar medium" published in Nature Astronomy.

The team is composed of M. Franco (Centre for Astrophysics Research, University of Hertfordshire, UK [CAR]), K. E. K. Coppin (CAR), J. E. Geach (CAR), C. Kobayashi (CAR, Leverhulme Research Fellow), S. C. Chapman (Department of Physics and Atmospheric Science, Dalhousie University, Canada and National Research Council, Herzberg Astronomy and Astrophysics, Canada), C. Yang (European Southern Observatory, Chile), E. González-Alfonso (Universidad de Alcalá, Departamento de Física y Matematicas, Spain), J. S. Spilker (Department of Astronomy, University of Texas at Austin, USA), A. Cooray (Department of Physics and Astronomy, University of California, Irvine, USA), M. J. Michałowski (Astronomical Observatory Institute, Faculty of Physics, Poland)


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