James Webb révèle la présence d’oxygène dans les galaxies premières

James Webb révèle la présence d'oxygène dans les galaxies premières

Scientists from around the world have analyzed the first data collected by the James Webb Space Telescope unveiled on July 12. A team from Geneva shows that the oldest galaxies are much more evolved than models predicted. And it’s a surprise!

Scientists at the Geneva Observatory have examined around a hundred very old galaxies detected in images taken by the James Webb Space Telescope (JWST). Galaxies so close to the time of the Big Bang, a foundational event that took place 13.8 billion years ago…

For the first time, the team has identified the chemical composition of two of them, visible between 600 and 700 million years after the Big Bang, those framed in yellow in the image that heads the article. His study was prepublished on the arXiv websitein order to be available to the scientific community.

“The first surprise is the spectacular oxygen signature in the first JWST observations,” enthuses the study’s first author, Daniel Schaerer of the Geneva Observatory. “The data show that these distant galaxies are already much more evolved than predicted by computer-generated astrophysical models,” he adds, responding to RTSinfo by phone.

Scientists are delighted with this unexpected discovery. Unheard of: Hubble was unable to measure the spectra of such distant galaxies, which was made possible by the tremendously powerful spectrography instruments on board JWST, including the NIRSpec instrument. (read box). This measures the spectral shift of the target object: the spectrum obtained gives information about the age and chemical composition of the galaxies.

>> The emission spectrum taken by NIRSpec of a galaxy located 13.1 billion years ago:

The first spectroscopy of the composition of a distant galaxy given by the NIRSpec instrument on the James Webb Space Telescope. [NIRSpec Emission Spectrum/JWST – NASA, ESA, CSA, STScI]

>> The same spectrum in a more scientific version, used in the published study:

The spectrum of a galaxy obtained with NIRSpec.  Vertical dashed lines mark the position of well-detected nebular emission lines. [Daniel Schaerer et al., 22 juillet 2022 - Astronomy & Astrophysics]The spectrum of a galaxy obtained with NIRSpec. Vertical dashed lines mark the position of well-detected nebular emission lines. [Daniel Schaerer et al., 22 juillet 2022 – Astronomy & Astrophysics]

A history of chemistry.

The most common chemical elements in the Universe -the lightest and also the simplest- are hydrogen and helium: “When these gases fuse inside stars, they end up producing more complex elements -and heavier- such as carbon, nitrogen, oxygen or neon. ”, explains Prof. Schaerer.

Scientists were not expecting these elements some 600 million years after the Big Bang: “In a galaxy that has not evolved much, there have not yet been many generations of stars… and these are the massive ones that produce the heavy chemical elements. They expel them: the oxygen produced, for example, then goes on to the next generation of stars.”

“In such a distant time, the chemical composition had never been measured!” emphasizes the astrophysicist. “In addition, the abundance of oxygen -about ten times less than in the Sun- shows that matter has been recycled very quickly. These galaxies evolve very fast, giving life to many stars. Stars that have a very short life: this is how they produce oxygen.”

An evolution to clarify

From now on, scientists will have to work to understand how the chemical composition of these distant galaxies evolves and how quickly the different heavier chemical elements appear.

“Heavy elements are a sign of production by stars. When we see carbon, nitrogen, oxygen, we know that there has been a lot of stellar activity. A low-mass star like our Sun lives for nine billion years: it evolves slowly. Therefore, it has little effect on the chemical composition of its galaxy”, Daniel Schaerer also indicates.

“It’s the short-lived stars that produce oxygen. And it takes a lot of it to start a new cycle of star generation.”

Stephanie Jaquet

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