Astronomers have detected polycyclic aromatic hydrocarbons (PAHs) in a galaxy observed as it was less than 1.5 billion years after the Big Bang, according to a new report (Nature 2023, DOI: 10.1038/s41586-023-05998-6). The findings demonstrate the James Webb Space Telescope’s (JWST) prowess for probing chemistry in deep time.
While earthbound chemists might find these complex organic molecules in soot or smog, astronomers seek out PAHs in space to guide them toward newborn stars. When stars first form, they blast out intense ultraviolet radiation. That UV light photoactivates PAHs that emit infrared (IR) radiation in turn. So when IR telescopes spot PAHs emissions, it’s likely because blazing young star lit them up, says Justin Spilker, an astrophysicist at Texas A&M University. That’s what the Spitzer Space Telescope and othershave observed in nearby galaxies, but limited instrument sensitivity has prevented astronomers from glimpsing PAHs in our universe’s deep past, he says. With the promise of JWST’s next-generation IR spectrometers, Spilker and his colleagues saw an opportunity to test both the telescope’s ability to sniff out PAHs and PAHs’ suitability as proxies for star formation.
To begin, Spilker and his colleagues trained JWST on a galaxy called SPT0418-47 that lies about 12 billion light years away. Viewed from Earth, this galaxy appears partially obscured by a second galaxy only 3 billion light years away. Called an Einstein ring (shown), this near-perfect galactic alignment allows the gravitational field of the nearer system to distort and magnify light from SPT0418-47 through an optical phenomenon called lensing. Thanks to this lensing, astronomers get a view of SPT0418-47 as it appeared when the universe was less than 1.5 billion years old. “The universe is basically a baby at that time,” Spilker says. Scientists previously identified regions of star formation in SPT0418-47, which is large compared to other galaxies in the infant universe, he says. Now the JWST Mid-Infrared Instrument (MIRI) has also detected diagnostic spectroscopic signatures of PAHs in the interstellar dust.
Spilker and his team were surprised to discover that the presence of PAHs in SPT0418-47 did not consistently overlap with known star formation activity. “We have a lot of regions where we have star formation, but don’t have any of these molecules. And it’s not really clear why that is yet,” Spilker says. This new mystery will require further investigation, he says. Now that his team has shown JWST is up to the task, Spilker hopes to look for PAHs even deeper in the past.
This is the furthest back in time that we’ve seen PAHs in space, says Alexander Tielens, an astrophysical chemist at Leiden University and the University of Maryland. Tielens, who was not involved in the study, is intrigued by the mismatch between PAHs signals and star formation activity in SPT0418-47 and agrees the observation warrants further investigation. He also points to new questions about PAHs themselves. “The origin of these molecules is not very clear,” Tielens says. “That you already have PAHs that early means that [the universe has] an efficient way of producing it.”
This story was updated on June 8, 2023 to correct an error in a reference to the galaxy SPT0418-47.