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Complex Organic Molecules Found in a Galaxy at the Birth of the Universe

In a galaxy hanging out in the early Universe less than 1.5 billion years after the Big Bang, the James Webb Space Telescope has made an astonishing detection.

From light that traveled for over 12 billion years from a galaxy known as SPT0418-47, astronomers teased out the spectral signal of complex molecules – the polycyclic aromatic hydrocarbons (PAHs) that make up some of the dust grains in the clouds that drift between the stars, soaking up the light and re-emitting it at infrared wavelengths.

“Dust grains absorb half of the radiation emitted by stars throughout the history of the universe, re-emitting this energy at infrared wavelengths,” said University of Illinois Urbana-Champaign’s Professor Joaquin Vieira and colleagues.

“Polycyclic aromatic hydrocarbons (PAHs) are large organic molecules that trace millimeter-size dust grains and regulate the cooling of interstellar gas within galaxies.”

“Observations of PAH features in very distant galaxies have been difficult owing to the limited sensitivity and wavelength coverage of previous infrared telescopes.”

In their research, the astronomers used Webb to observe the dust-obscured galaxy SPT0418-47.

This galaxy is 12 billion light-years from Earth, corresponding to a time when the Universe was less than 1.5 billion years old, or about 10% of its current age.

It is magnified by a factor of roughly 30-35 by strong gravitational lensing into a hallmark Einstein ring morphology by a foreground galaxy.

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“Before having access to the combined power of gravitational lensing and Webb, we could neither see nor spatially resolve the actual background galaxy through all of the dust,” Professor Vieira said.

The spectroscopic data from Webb suggest that the obscured interstellar gas in SPT0418-47 is enriched in heavy elements, indicating that generations of stars have already lived and died.

The high equivalent width of the PAH feature indicates that star formation, rather than black hole accretion, dominates infrared emission throughout the galaxy.

“What this research is telling us right now, and we are still learning, is that we can see all of the regions where these smaller dust grains are located — regions that we could never see before Webb,” said University of Illinois Urbana-Champaign graduate student Kedar Phadke.

“The new spectroscopic data lets us observe the galaxy’s atomic and molecular composition, providing very important insights into the formation of galaxies, their lifecycle and how they evolve.”

“We didn’t expect this,” Professor Vieira added.

“Detecting these complex organic molecules at such a vast distance is game-changing regarding future observations.”

“This work is just the first step, and we’re just now learning how to use it and learn its capabilities.”

“We are very excited to see how this plays out.”


J.S. Spilker et al. Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy. Nature, published online June 5, 2023; doi: 10.1038/s41586-023-05998-6


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