Astrochemistry

Abundant Hydrocarbons In A Buried Galactic Nucleus With Signs Of Carbonaceous Grain And Polycyclic Aromatic Hydrocarbon Processing

By Keith Cowing
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astro-ph.GA
February 6, 2026
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Abundant Hydrocarbons In A Buried Galactic Nucleus With Signs Of Carbonaceous Grain And Polycyclic Aromatic Hydrocarbon Processing
Nuclear dust and molecular structure, with a sketch summarizing grain and PAH processing in IRAS 07251−0248. Left panel: A schematic view of the central region of IRAS 07251−0248. The color gradient represents decreasing temperature, from the extremely compact hot component (dark red; r<20 pc), through the warm shell containing gas-phase molecules (orange-yellow; r∼70 pc Gonz´alez-Alfonso et al. in prep.), to the colder and icy envelope where solid-phase molecules reside (blue-gray). Right panel: Conceptual sketch illustrating the proposed scenario of carbonaceous grains and PAH processing driven by cosmic rays, which are responsible for hydrocarbon-rich chemistry. JWST icon credit: NASA Science Multimedia, https://science.nasa.gov/wp-content/uploads/2023/ 05/webb 2.png — astro-ph.GA

Hydrocarbons play a key role in shaping the chemistry of the interstellar medium (ISM), but their enrichment and relationship with carbonaceous grains and polycyclic aromatic hydrocarbons (PAHs) still lack clear observational constraints.

We report JWST NIRSpec+MIRI/MRS infrared (IR; 3-28 micron) observations of the local ultra-luminous IR galaxy (ULIRG) IRAS 07251-0248, revealing the extragalactic detection of small gas-phase hydrocarbons such as benzene (C6H6), triacetylene (C6H2), diacetylene (C4H2), acetylene (C2H2), methane (CH4), and methyl radical (CH3) as well as deep amorphous C-H absorptions in the solid phase.

The unexpectedly high abundance of these molecules indicates an extremely rich hydrocarbon chemistry, not explained by high-temperature gas-phase chemistry, ice desorption or oxygen depletion. Instead, the most plausible explanation is the erosion and fragmentation of carbonaceous grains and PAHs.

This scenario is supported by the correlation between the abundance of one of their main fragmentation products, C2H2, and cosmic ray (CR) ionization rate for a sample of local ULIRGs. These hydrocarbons are outflowing at ∼160 km/s, which may represent a potential formation pathway for hydrogenated amorphous grains.

Our results suggest that IRAS 07251-0248 might not be unique but represents an extreme example of the commonly rich hydrocarbon chemistry prevalent in deeply obscured galactic nuclei.

I. García-Bernete (1), M. Pereira-Santaella (2), E. González-Alfonso (3), M. Agúndez (2), D. Rigopoulou (4 and 5), F.R. Donnan (4), G. Speranza (2), N. Thatte (4) ((1) Centro de Astrobiología (CAB, CSIC-INTA), (2) Instituto de Física Fundamental (IFF, CSIC), (3) Departamento de Física y Matemáticas, Universidad de Alcalá, (4) Department of Physics, University of Oxford, (5) School of Sciences, European University Cyprus)

Comments: To appear in Nature Astronomy. 15 Figures, 1 Table
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2602.04967 [astro-ph.GA] (or arXiv:2602.04967v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2602.04967
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Related DOI:
https://doi.org/10.1038/s41550-025-02750-0
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Submission history
From: Ismael Garcia-Bernete Dr.
[v1] Wed, 4 Feb 2026 19:00:06 UTC (2,584 KB)
https://arxiv.org/abs/2602.04967
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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