A new discovery is shedding light on how fluorine — an element found in our bones and teeth — forms in the Universe. With the help of the Atacama Large Millimeter/submillimeter Array, in which ESO is a partner, a team of astronomers has detected this element in a galaxy that is so far away that its light takes more than 12 billion years to reach us. This is the first time that fluorine has been discovered in a galaxy with star formation so far away.
“We all know fluoride because the toothpaste we use every day contains it,” explains Maximilien Franco of the University of Hertfordshire in the UK, who led this new study published today in the journal Nature Astronomy. Like most elements around us, fluorine is created inside stars, but until now, we didn’t know exactly how this element formed. “We didn’t even know what kind of stars gave rise to most of the fluor in the Universe!”
Franco and colleagues discovered fluorine (in the form of hydrogen fluoride) in the huge gas clouds of the distant galaxy NGP-190387, which we observed when the Universe was only 1.4 billion years old, or about 10% of your current age. Since stars expel the elements that form in their cores when they reach the end of their lives, this detection implies that the stars that formed the fluorine must have lived and died very quickly.
The team thinks that Wolf-Rayet stars, very massive stars with a lifetime of just a few million years, which is a blink of an eye in the history of the Universe, are the most likely sites for fluorine production. Scientists think these stars are necessary to explain the huge amounts of hydrogen fluoride discovered by the team. Wolf-Rayet stars have previously been suggested as possible sources of cosmic fluorine, however, until now, astronomers did not know how important they were in producing this element in the early Universe.
“We found that Wolf-Rayet stars, which are among the most massive we know and can explode violently when they reach the end of their lives, help us, in a way, to maintain good oral hygiene!” exclaims Franco playfully.
In addition to these stars, other scenarios have also emerged in the past to explain how fluorine is produced and expelled, such as the pulsations of evolved giant stars with masses sometimes even that of our Sun, the so-called stars in the asymtotics branch giants. However, the team believes that these scenarios, some of which last for billions of years, may not fully explain the amount of fluorine we see in NGP-190387.
“This galaxy took just a few 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 is a completely unexpected result”, explains Chiaki Kobayashi, Professor at the University of Hertfordshire. “Our measurements put new limits on the origin of fluorine, which we have been studying for two decades now.”
This discovery in NGP-190387 marks one of the first detections of fluorine outside the Milky Way and neighboring galaxies. Astronomers have previously detected this element in distant quasars, bright objects fed by supermassive black holes located at the center of some galaxies. However, and until now, fluorine has never been observed in a star-forming galaxy so early in the history of the Universe.
The detection of fluorine by the team was a chance discovery and made possible by the use of observatories placed on the ground and in space. NGP-190387, originally discovered by the European Space Agency’s Herschel Space Observatory and later observed with ALMA in Chile, is extraordinarily bright for its distance. ALMA data confirmed that the exceptional luminosity of NGP-190387 is in part caused by another known massive galaxy, located between NGP-190387 and Earth, very close to our line of sight. This massive galaxy amplified the light observed by Franco and his team, allowing them to identify the faint radiation emitted billions of years ago by fluorine from NGP-190387.
Future studies of NGP-190387 with the Extremely Large Telescope (ELT) — ESO’s new flagship project under construction in Chile and slated to start operating later this decade — could reveal more secrets about this galaxy. “ALMA is sensitive to radiation emitted by cold interstellar gas and dust,” says Chentao Yang, an ESO fellow in Chile. “With the ELT we will be able to observe NGP-190387 through direct starlight, which will give us crucial information about the stellar content of this galaxy.”