Fragrances of Cosmic Noon: How PAHs Survive the Hell of a Double Quasar
Using the infrared spectrometer of the James Webb Space Telescope, scientists have detected emission from both aromatic (ring-shaped) and aliphatic (chain-like) hydrocarbons at wavelengths of 3.3 and 3.4 µm in the double quasar J0749+2255 (when the Universe was ~3 billion years old). Aliphatic fragments, like organic "hairs" on carbon "skeletons", turned out to be unexpectedly abundant in the brightest regions near the supermassive black holes. This means that even small PAHs (polycyclic aromatic hydrocarbons, about 20–30 carbon atoms) and their side chains can withstand the extreme ultraviolet storm from stellar flares. The discovery broadens our understanding of how complex organics survived in the early Universe.
The era of cosmic noon is a time when the Universe was experiencing its stormy youth. Three billion years after the Big Bang, galaxies were giving birth to stars at a frantic pace, and supermassive black holes were growing in their centers. During this turbulent period, binary systems like J0749+2255 were born — two galaxies merging into one, each with its own central monster. Thanks to the work of Edwin Hubble and Georges Lemaître, we know that light from this pair traveled to us for over 10 billion years, traversing the expanding space. And so, by pointing the MIRI infrared spectrograph aboard JWST at the distant system, astronomers sniffed out something astonishing — the scent of organic molecules.
These molecules — polycyclic aromatic hydrocarbons, or PAHs — are true cosmic perfumes. Imagine a bottle of complex perfume: it has persistent base notes, like large carbon rings, and volatile top notes — aliphatic chains attached to the rings. In the same way, PAHs consist of a carbon skeleton bristling with hydrogen "tufts". When black holes and young stars irradiate them with ultraviolet light, the molecules begin to glow in the infrared, revealing their composition. Using spectroscopy, the MIRI instrument split this light into its components, and scientists, applying the PAHFIT code, identified two characteristic bands: at 3.3 and 3.4 micrometers. The first corresponds to aromatic carbon-hydrogen bonds, the second to aliphatic ones. It was expected that near the cores, where radiation is especially harsh, the aliphatic tails would be destroyed, like a delicate scent in a flame. But reality turned out differently.
The aliphatic fraction — the share of carbon in side groups — reached 14% in one of the brightest spots on the core. For comparison, in the famous "starburst" galaxy M82, this figure is only 8.4%. One might think that the farther from the center, the safer for delicate molecules, but the data showed an opposite trend: in the outer regions, the fraction is on average lower and fluctuates from zero to 15%. Moreover, the PAH emission outlined mysterious structures resembling tidal tails and spiral arms, invisible in Hubble images. Thus, cosmic perfumes revealed the hidden pattern of the dance of merging galaxies, like invisible ink on an astrophysicist's map.
This discovery undermines the long-held belief that small PAHs cannot survive near active nuclei. Either the destructive action of ultraviolet radiation and shock waves is overestimated, or there are unknown mechanisms that rebuild the molecules right after bond breakage. It's as if a perfume miraculously restored its scent after being burned. For cosmology, this means that chemical networks in galaxy evolution simulations need revision, and PAHs become a powerful diagnostic tool for mergers. In the future, James Webb and the ground-based giant ELT will enable detailed "aromatic tomography" of such systems, separating the contributions of stellar nurseries and black holes. We stand on the threshold of an era when the scent of galaxies will tell us how worlds are assembled from cosmic dust.
🎯 Polycyclic aromatic hydrocarbons are not just cosmic exotica, but also familiar earthly grime. They form during incomplete combustion of organic matter and are found in exhaust gases and soot. Some PAHs have recognizable odors: for instance, naphthalene smells like mothballs.