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

TL;DR

The paper reports JWST observations of the deeply buried ULIRG IRAS 07251-0248 that reveal an unexpectedly rich inventory of small gas-phase hydrocarbons (e.g., C$_6$H$_6$, C$_6$H$_2$, C$_4$H$_2$, C$_2$H$_2$, CH$_4$, CH$_3$) and deep solid-phase carbon features, in a nucleus obscured by dust. Through LTE radiative-transfer modelling of the gas-phase bands and detailed solid-phase fits, the authors derive warm ($T_{\rm kin}\sim150-250$ K) gas with $N_{\rm H}\sim2\times10^{23}$ cm$^{-2}$ and an outflow velocity of $v\sim160$ km s$^{-1}$, accompanied by abundant hydrogenated amorphous carbon grains. Standard high-temperature gas-phase chemistry, ice-desorption, or oxygen-depletion scenarios struggle to reproduce the observed hydrocarbon abundances, suggesting an additional carbon source. The authors propose that erosion and fragmentation of carbonaceous grains and PAHs—facilitated by cosmic-ray processing—releases small hydrocarbons into the gas phase, a scenario supported by correlations between C$_2$H$_2$ abundance and cosmic-ray ionization indicators across local ULIRGs; this mechanism may reflect a general chemical pathway in deeply obscured galactic nuclei.

Abstract

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.

Figures (15)

• Figure 1: JWST near- and mid-IR spectrum of the eastern nucleus of IRAS 07251$-$0248. The spectrum was extracted assuming it is a point source ($\sim$0.4" aperture diameter at 10 $\mu$m; see Methods for further details). The main molecular gas-phase bands detected in this source are indicated: CO, CO$_2$, H$_2$O, HCN, CH$_4$ and C$_2$H$_2$. The vertical green dotted lines correspond to the clearly detected PAH bands in IRAS 07251$-$0248. The top shaded regions represent the typical extent of the ices and dust features.
• Figure 1: Fitted baseline of IRAS 07251$-$0248. The JWST rest-frame spectra and baseline fit correspond to the solid black and red lines.
• Figure 2: Main mid-IR gas-phase molecular rovibrational bands in IRAS 07251$-$0248. The top panel shows the $\sim$7-8 $\mu$m molecular bands and the middle and bottom panels the $\sim$13-18 $\mu$m bands. Each panel shows the JWST/MIRI-MRS rest-frame continuum-normalized spectra (Extended data Fig. 1; black solid line filled in gray) together with the total best-fit model (red solid line). The contribution of the fit by the different species are shown with an offset ($+$0.15) to improve clarity: C$_6$H$_6$ (dark red), C$_6$H$_2$ (blue), C$_4$H$_2$ (deep pink), CH$_4$ (magenta), CH$_3$ (green), C$_2$H$_2$ (brown), CO$_2$ (light pink) and HCN (orange).
• Figure 2: H$_2$O $v_2$=1-0 (R-branch $\sim$5.3-6.2 $\mu$m) gas-phase molecular rovibrational bands in IRAS 07251-0248. The JWST/MIRI-MRS rest-frame continuum-normalized spectra (black solid line; filled in gray) is shown together with the total best-fit model (red solid line). The contribution of the fit (solid blue line) is shown with an offset ($+$0.15) to improve clarity.
• Figure 3: Relationship between key PAH flux ratios in deeply obscured (U)LIRGs. Comparison of the 11.3/3.3 $\mu$m and 11.3/6.2 $\mu$m PAH flux ratios for the sample analyzed in this work. Error bars indicate 1$\sigma$ uncertainties.