Abundant hydrocarbons in a buried galactic nucleus with signs of carbonaceous grain and polycyclic aromatic hydrocarbon processing

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 (C$_6$H$_6$), triacetylene (C$_6$H$_2$), diacetylene (C$_4$H$_2$), acetylene (C$_2$H$_2$), methane (CH$_4$), and methyl radical (CH$_3$) 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, C$_2$H$_2$, and cosmic ray (CR) ionization rate for a sample of local ULIRGs. These hydrocarbons are outflowing at $\sim$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.

💡 Research Summary

The authors present JWST NIRSpec + MIRI/MRS observations of the heavily obscured nucleus of the local ultra‑luminous infrared galaxy IRAS 07251‑0248, covering the 3–28 µm infrared range with high spectral resolution. The spectrum exhibits deep silicate, water‑ice, and amorphous carbon (a‑C:H) absorption features, together with the classic molecular bands of CO, ¹³CO, H₂O, HCN, C₂H₂, and CO₂. In addition, they detect for the first time in an extragalactic source a suite of small gas‑phase hydrocarbons: benzene (C₆H₆), triacetylene (C₆H₂), diacetylene (C₄H₂), acetylene (C₂H₂), methane (CH₄), and the methyl radical (CH₃). These molecules appear in absorption, allowing the authors to model them with LTE radiative‑transfer calculations, derive rotational temperatures of 150–250 K, and obtain column densities up to 10¹⁸ cm⁻² (e.g., N(C₆H₆) ≈ 3.7 × 10¹⁶ cm⁻², N(C₂H₂) ≈ 1.9 × 10¹⁸ cm⁻²). A covering factor of ~0.6–0.7 is required to account for partial continuum filling.

PAH emission is also analyzed. The 3.3 µm and 11.3 µm bands (neutral PAH) are strong, whereas the ionized PAH bands at 6.2, 7.7, 8.6 µm are heavily diluted. The 11.3/3.3 and 11.3/6.2 flux ratios are markedly elevated, indicating a PAH population dominated by large, neutral molecules. This is consistent with a scenario where PAHs are being broken down, feeding the observed small hydrocarbons.

The authors explore three possible origins for the unusually high hydrocarbon abundances. High‑temperature gas‑phase chemistry (>500 K) can boost many species but still underproduces CH₄ by two orders of magnitude and cannot explain the observed rotational temperatures. An oxygen‑depletion scenario that raises the gas‑phase C/O ratio to ~1 also fails to reproduce the full set of abundances. The most plausible explanation is the erosion and fragmentation of carbonaceous grains and PAHs under intense cosmic‑ray irradiation. Chemical modeling that includes a high cosmic‑ray ionization rate (derived from H₃⁺ measurements) reproduces the observed correlation between C₂H₂ column density and ζ_CR across a sample of local ULIRGs. The hydrocarbons are observed in outflowing gas at ~160 km s⁻¹, suggesting that grain fragmentation products are being expelled and may contribute to the formation of hydrogenated amorphous carbon grains in the outflow.

From the molecular inventory the authors infer a gas‑phase C/O ratio of ~1.03, using CO, H₂O, and the detected hydrocarbons. This carbon‑rich environment, together with the deep a‑C:H absorption, points to a “hydrocarbon‑rich” nucleus where organic molecules are as abundant as, or more abundant than, water. The study concludes that IRAS 07251‑0248 is an extreme but not unique example of a buried galactic nucleus where grain processing, driven by strong cosmic‑ray fields, dominates the chemistry, injecting a rich suite of small hydrocarbons into the ISM. This finding has broad implications for our understanding of dust evolution, PAH life cycles, and organic chemistry in the most obscured regions of galaxies.