Aromatic Species In The Molecular Universe

Interstellar polycyclic aromatic hydrocarbon (PAHs) are an important component of the interstellar medium of galaxies, containing some 10 percent of the elemental carbon. Their vibrational emission dominates the mid-infrared spectra of galactic and extragalactic objects.

PAHs control the heating of interstellar neutral gas and the charge balance of molecular clouds. PAHs are formed in the outflows from late type stars through chemical processes akin to those in sooting flames and then further processed in the interstellar medium by UV photolysis and strong shock waves. PAHs are also formed through ion molecule reactions and neutral radical reactions in dense cloud cores.

The James Webb Space Telescope has provided a wealth of high-quality spectra that have provided new insights in the characteristics of the interstellar PAH family. Their analysis is supported by dedicated laboratory and quantum chemistry studies, feeding into detailed molecular physics models relevant to astronomical environments. Laboratory studies have also provided deeper insight in the chemical evolution of PAHs in the interstellar medium. This paper will review progress in the field and chart its future.

Top: Left: false color NIRCam image of the Orion Bar11 showing the NIRSpec mosaic footprint (white boundary). The composite image combines AIB emission (red), H2 emission (green), and H I Paschen α (blue). Top Right: Blow up of the area mapped spectrally with NIRSpec and MIRI. The black boxes define the apertures where the spectra shown in the bottom panel were extracted.12 DF1−3 indicate the positions of the H2 dissociation fronts. The location of two proplyds in the Orion Bar are marked by small circles.

Bottom: the incredibly rich, Aromatic Infrared Band (AIB) spectrum as observed in the Orion Bar.13 A continuum was subtracted to highlight the AIBs. The different colored spectra refer to templates for five distinct regionsin the Orion PDR, the (background PDR behind the) H II region, the atomic zone in the PDR, and the three dissociation frontsthat traverse the region spectrally imaged. Each spectral window (on an Fν scale) is normalized by the peak surface brightness of the indicated AIB on the yaxes in each panel. The vertical tick marks indicate the positions of identified (blue) or tentative (black) AIBs and components. Red dashed vertical ticks indicate the wavelengths where the data switches from one MIRI subband to the next. Figure taken from.13 Reproduced with permission from Astronomy & Astrophysics, Ⓒ ESO. — astro-ph.GA

A.G.G.M. Tielens

Comments: Earth and Space Chemistry (2026)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2603.04536 [astro-ph.GA](or arXiv:2603.04536v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2603.04536
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Related DOI:
https://doi.org/10.1021/acsearthspacechem.5c00385
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Submission history
From: Alexander Tielens
[v1] Wed, 4 Mar 2026 19:23:23 UTC (9,491 KB)
https://arxiv.org/abs/2603.04536

Astrobiology, Astrochemistry,