A new discovery sheds light on how fluorine – an element found as fluoride in our bones and teeth – is forged in the universe. With the Atacama Large Millimeter / submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is involved, a team of astronomers has proven this element in a galaxy so distant that its light took over 12 billion years to reach us. This is the first time fluorine has been seen in a star-forming galaxy this far away.
“We all know fluorine because the toothpaste we use every day contains it in the form of fluoride,” says Maximilien Franco of the University of Hertfordshire in the UK, who led the new study published today in Nature Astronomy. Like most of the elements around us, fluorine is formed inside stars, but until now we did not know exactly how this element was made. “We didn’t even know what kind of stars produced most of the fluorine in the universe!”
Franco and his co-workers discovered fluorine (in the form of hydrogen fluoride) in the great gas clouds of the distant galaxy NGP-190387 that we see when the universe was only 1.4 billion years old, about 10% of its current state, the age. Since stars emit the elements that make up their core when they reach the end of their life, this evidence implies that the stars that produced fluorine must have lived and died quickly.
The team believes that Wolf-Rayet stars, very massive stars that are only a few million years old, are the blink of an eye in the history of the universe, are the most likely production sites for fluorine. They are needed to explain the amounts of hydrogen fluoride the team discovered, they say. Wolf-Rayet stars have been suggested as possible sources of cosmic fluorine, but until now astronomers did not know how important they were in the manufacture of this element in the early Universe.
“We have shown that Wolf-Rayet stars, which are some of the most massive stars known and can explode violently at the end of their life, help us in some ways to maintain good dental health!” jokes Franco.
In addition to these stars, other scenarios for the production and excretion of fluorine have been set up in the past. One example is the pulsations of giant, evolved stars with masses that are less than the mass of our Sun, known as asymptotic giant twig stars. However, the team believes that these scenarios, some of which take billions of years, may not fully explain the amount of fluorine in NGP-190387.
“It only took dozens or hundreds of millions of years for this galaxy to achieve a fluorine content comparable to that in the 13.5 billion year old stars of the Milky Way. This was a completely unexpected result, ”says Chiaki Kobayashi, a. Professor at the University of Hertfordshire. “Our measurement adds an entirely new restriction on the origin of fluorine that has been studied for two decades.”
The discovery in NGP-190387 is one of the earliest discoveries of fluorine beyond the Milky Way and its neighboring galaxies. Astronomers previously discovered this element in distant quasars, bright objects propelled by supermassive black holes in the center of some galaxies. But never before in the history of the universe had this element been observed in a star-forming galaxy.
The team’s detection of fluorine was an accidental 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 Chile-based ALMA, is exceptionally bright for its distance. The ALMA data confirmed that the extraordinary luminosity of NGP-190387 was partially due to caused another known massive galaxy located between NGP-190387 and Earth in close proximity to the line of sight. This massive galaxy amplified the light observed by Franco and his coworkers and enabled them to see the faint radiation emitted billions of years ago by the fluorine in NGP-190387.
Future studies of NGP-190387 with the Extremely Large Telescope (ELT) – ESO’s new flagship project under construction in Chile and scheduled to become operational later this decade – could reveal further secrets of this galaxy. “ALMA is sensitive to radiation from cold interstellar gas and dust,” says Chentao Yang, ESO Fellow in Chile. “With the ELT we will be able to observe NGP-190387 through the direct light of stars and thus gain important information about the stellar content of this galaxy.”
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