Aromas of Cosmic Noon: PAHs in a Binary Quasar at the Edge of the Universe
Using integral field spectroscopy with JWST/MIRI, in the double quasar SDSSJ074922.96+225511.7 at redshift z~2.17 (corresponding to a Universe age of about 3 billion years, the peak of star formation and black hole growth—"cosmic noon"), we have for the first time recorded joint emission from aromatic (3.3 µm, C–H stretching vibrations in small PAHs) and aliphatic (3.4 µm, C–H stretches of aliphatic side groups) hydrocarbons. From measured band intensities, the aliphatic fraction of PAHs and its spatial variations were derived. It turns out that in the brightest regions, concentrated around the nuclei of the two quasars, the aliphatic fraction is anomalously high. This indicates the survival of not only small PAHs (~20–30 carbon atoms) but also their attached aliphatic chains under powerful ultraviolet radiation from extreme star formation outbursts.
Context
The cosmic noon epoch, which occurred about 3 billion years after the Big Bang, was a key period in the universe's history when star formation rates and the growth of supermassive black holes peaked. Studying dual quasars—rare systems where two such black holes are in the process of merging—offers a glimpse into the co-evolution mechanisms of galaxies and their central monsters. Before the launch of JWST, astronomers couldn't detect the faint infrared emission from PAHs in such distant objects, and only now can we test how extreme conditions—powerful ultraviolet radiation and shock waves from quasars—affect the survival of interstellar molecules. These tiny carbon-based particles serve as sensitive tracers of physical conditions, and their surprising behavior may reshape our understanding of galactic chemical evolution. Pioneering work by Edwin Hubble on redshift measurements and theoretical predictions by Georges Lemaître about an expanding universe laid the groundwork for modern cosmology, without which interpreting observations of distant objects like J0749+2255 would be impossible.
Methods
The researchers used integral field spectroscopy (IFU) data from the MIRI instrument on the JWST, acquired in 2022. Images from the Hubble Space Telescope (WFC3 camera) were also used to create a color composite image of the system. Spectra were extracted via aperture photometry for 47 regions: 4 small apertures with a diameter of 1.7 kpc, aimed at the nuclei and surroundings; one large elliptical aperture covering the entire central area; and 42 apertures of 6.6 kpc each, covering the outer part of the system. For emission line analysis, spectral decomposition was performed using the PAHFIT code, which models the observed spectrum as the sum of two Drude profiles (characterizing the 3.3 and 3.4 µm bands) and an underlying continuum. Line intensities reflect vibrational modes of carbon-hydrogen bonds in aromatic rings and aliphatic chains, allowing measurement of the luminosity in each band and calculation of the PAH aliphatic fraction—the proportion of carbon atoms in aliphatic groups relative to the total number of carbon atoms.
Results
The headline result is the ubiquitous detection of both aromatic (3.3 µm) and aliphatic (3.4 µm) emission across the entire dual quasar system. This is the most distant galaxy where both bands have been observed simultaneously: a redshift of 2.17 corresponds to an age of the universe of about 3 billion years. In the central region, the aliphatic fraction turned out to be unexpectedly high: between 8% and 14%, with the maximum value recorded in one of the brightest apertures on the quasar core, where the supermassive black hole dominates. For comparison, in the nearby starburst galaxy M82 this value is about 8.4%. In the outer regions, the aliphatic fraction is on average lower and shows considerable scatter: from 0% to 15.2%, with only four out of 20 apertures exceeding 10%. Interestingly, there's a trend of decreasing aliphatic fraction with distance from the center, which runs counter to expectations since the intensity of destructive UV radiation and quasar-driven shocks should weaken with distance. Moreover, PAH emission unexpectedly revealed structures reminiscent of tidal tails or spiral arms, invisible in ordinary images, hinting at hidden details of the galaxy merger process.
Implications
These findings challenge established notions about the chemical resilience of PAHs in harsh environments. It was thought that small PAHs (containing 20–30 carbon atoms) and their aliphatic side groups should not survive near active nuclei, where intense radiation and shocks would destroy them. Yet JWST data show the opposite: even in the most energy-dense regions, PAHs not only persist but also boast a high proportion of aliphatic fragments. This means either destruction processes are less effective on kiloparsec scales, or there are mechanisms for rapid repair or shielding of the molecules. Additionally, the spatial distribution of PAH emission provides a new way to map dynamic structures in merging galaxies, much like molecular hydrogen or ionized gas are used to trace shocks.
Future development
This work opens up several promising directions. First, detailed laboratory and theoretical studies are needed on the excitation and destruction mechanisms of PAHs under conditions similar to those observed, to understand how small molecules survive near quasars. Second, future observations by JWST and next-generation telescopes like the Extremely Large Telescope (ELT) will deliver spectra with higher spatial resolution and sensitivity, enabling separation of the contributions from star formation and supermassive black hole activity. Third, a statistical analysis of a sample of distant dual quasars will help establish whether this PAH behavior is typical or unique. Finally, isotopic measurements of carbon and hydrogen in these molecules could shed light on nucleosynthesis in the early universe.
Impact
The results will impact astrophysics of the interstellar medium, the theory of galaxy evolution, and the physics of active nuclei, as PAHs are proposed for use as a new diagnostic tool for studying merger processes and feedback from supermassive black holes. Moreover, laboratory astrochemistry will gain impetus for more accurate modeling of aromatic molecule behavior under extreme conditions.
Next steps
In the coming years, a similar analysis is planned for other dual quasars and starburst galaxies observed by JWST, with data compared to galaxy merger simulations. Laboratory experiments exposing PAHs to intense UV fluxes and shock waves are also expected to test hypotheses about survival mechanisms.
Key open problems
This study directly links to the unsolved problem of dust formation and evolution in galaxies, as well as the question of how supermassive black holes regulate star formation. The unexpected hardiness of PAHs near quasars casts doubt on current molecular destruction models and may require a revision of chemical networks used in cosmological simulations. Furthermore, the spatial distribution of PAH emission could help solve the mystery of the origin of tidal tails and their role in redistributing matter during mergers.
🎯 Polycyclic aromatic hydrocarbons (PAHs) are not just cosmic molecules—they're also common pollutants on Earth, formed during incomplete combustion of organic matter and found in soot and car exhaust. Some PAHs, like naphthalene, have a distinctive smell—so the 'aroma' of space could literally be sniffed, if only it weren't a vacuum!
Key numbers
- redshift: 2.17
- cosmic_age: 3 billion years after the Big Bang
- projected_separation: 3.8 kpc
- max_aliphatic_fraction: 14%
- outer_aperture_size: 6.6 kpc