Webb confirms long-standing galaxy model

JWST image of the grand design spiral galaxy NGC 628. Credit: NASA / ESA / CSA / Judy Schmidt (CC BY 2.0)
Perhaps the greatest tool astronomers have is the ability to go back in time. Because starlight takes time to reach us, astronomers can observe the history of the universe by capturing the light of distant galaxies.
This is why observatories like the James Webb Space Telescope (JWST) are so useful. This allows us to study in detail how galaxies form and evolve. As recent research shows, we are now at the point where observations can confirm long-standing models of galaxies.
This particular model concerns how galaxies become chemically enriched. In the early universe, the first stars were giant creatures without planets, as they were mostly just hydrogen and helium. They quickly die, spewing out heavier elements from which more complex stars and planets can form.
With each successive generation, more elements are added to the mix. But as galaxies foster star swarms ranging from blue supergiants to red dwarfs, which stars play the biggest role in chemical enrichment?
Some models claim it is the most massive star. This makes sense, since massive stars explode as supernovae when they die. They dump their concentrated outer layers deep into space, allowing material to mix together in large molecular clouds where new stars form. But about 20 years ago, another model argued that smaller, sun-like stars played a larger role.
Stars like the Sun cannot be killed by powerful explosions. Billions of years from now, the Sun will expand into a red giant star. In a desperate attempt to keep burning, the core of the Sun-like star heats up so much that helium fuses with it and its diffuse outer layer expands.
In the Hartsprung-Russell diagram, they are known as asymptotic giant bifurcation (AGB) stars. Although each AGB star may throw less material into interstellar space, they are much more common than massive stars. Therefore, this model argues that AGB stars play a larger role in galaxy enrichment.


The Cat’s Eye Nebula is the remnant of an AGB star. Credits: ESA, NASA, HEIC, Hubble Heritage Team, STScI/AURA
Both models have their advantages, but it would be difficult to prove that the AGB model is better than the Giant model. It’s easy to observe supernovae in galaxies billions of light years away. This is not the case with AGB Star. Thanks to JWST, we can now test AGB models.
The study, published in Nature Astronomy, used JWST to examine the spectra of three young galaxies. Because Webb’s NIRSpec camera can capture high-resolution infrared spectra, the research team was able to confirm not only the presence of specific elements, but also their relative abundances.
They found a strong presence of common carbon and oxygen bands in AGB debris, but also the presence of rarer elements such as vanadium and zirconium. Taken together, this points to a type of AGB star known as a thermally pulsed AGB (TP-AGB).
Many red giant stars enter a pulse phase at the end of their lives. The hot core causes the outer layers to expand, the object cools a little, gravity compresses the star a little, which heats the core, and starts the whole process over again. This study shows that TP-AGB is particularly efficient at concentrating galaxies, confirming a 20-year-old model.
Further information: Shiying Lu et al. Strong spectral signatures from asymptotic giant divergent stars in distant quiescent galaxies, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02391-9
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