Complex molecules detected in the ancient galaxy near the dawn of time


In a galaxy located 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 has traveled over 12 billion years from a galaxy known as SPT0418-47, astronomers have extracted the spectral signal of complex molecules, polycyclic aromatic hydrocarbons (PAHs) that make up some of the dust grains in drifting clouds. among stars, absorbing light and re-emitting it at infrared wavelengths.

This dust indicates a high rate of star formation, which is not unexpected for a galaxy from this early age of the Universe. But the dust isn’t evenly distributed, showing that this star formation can be mapped to different locations within the galaxy, according to a team led by astronomer Justin Spilker of Texas A&M University.

And the ability to make such a detailed observation of a galaxy so far away is truly mind-blowing.


“Here we present observations from the James Webb Space Telescope detecting the 3.3 micrometre PAH feature in a galaxy observed less than 1.5 billion years after the Big Bang. The high equivalent width of the PAH feature indicates that star formation , rather than black hole accretion, dominates infrared emission throughout the galaxy,” the researchers write.

“Our observations demonstrate that the differences in the emission of PAH molecules and large dust grains are a complex result of localized processes within early galaxies.”

Polycyclic aromatic hydrocarbons might look high-end, but they’re not particularly rare. Here on Earth they are as common as soot. Because I’m in the soot. They are a class of organic compounds that contain a ring of carbon atoms that can form during the compression and heating of organic matter. Coal contains PAHs; so does smoke, smog and crude oil.


Even the origins of PAHs can be non-organic; as far as we know, most PAHs in the Universe are not organic. And there are a lot of them out there.

Previous analysis suggests that about 15 percent of all carbon between stars in galaxies like ours is bound to PAHs. Most float among the stars as dust in the interstellar medium and are considered a fairly reliable tracer of star formation.

We have detected PAHs in other galaxies, but finding them in very distant galaxies is much more challenging. These molecules absorb light and re-emit it in infrared wavelengths, and previous infrared telescopes had very limited sensitivity and coverage. However, we now have the JWST, the most powerful space telescope ever built, strongest in infrared wavelengths.


Diagram illustrating gravitational lensing. (NASA, ESA and L. Calada)

But this is not enough by itself. JWST had to tap into a quirk of physics to make such a detailed observation: gravitational lensing. This is a gravitational curvature of spacetime that occurs around massive objects in the Universe. Imagine a bowling ball placed on a trampoline: the trampoline fabric deforms and stretches in response to mass.

Spacetime does something similar around massive objects like galaxies and galaxy clusters, but there’s a bonus. As space-time is warped and stretched, any light that passes through it is also warped, magnified, and sometimes duplicated. This means we can effectively use these lenses as a sort of cosmic magnifier, adding a lot of oomph to the power of our telescopes.

Between us and SPT0418-47 is another galaxy, about 3 billion light-years away, providing that lensing oomph. This means that when JWST took observations of the galaxy as part of the TEMPLATES Early Release Science program, it was able to obtain enough detail for Spilker and his colleagues to extract the spectral signature of light emitted by PAHs at a length d mid-infrared wave of 3.3 micrometres.


This constitutes the most distant detection to date of complex aromatic molecules, and while there is much we still don’t know, the reason for the uneven distribution of PAHs across the galaxy is unknown, bodes well for future studies of galaxy evolution. in the early Universe.

The research was published in Nature.

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