JWST reveals cosmic ray dominated chemistry in the local ULIRG IRAS 07251$-$0248

We analyse the ro-vibrational absorption bands of various molecular cations (HCO${}^{+}$, HCNH${}^{+}$, and N${}_2$H${}^{+}$) and neutral species (HCN, HNC, and HC${}_3$N) detected in the $\mathit{\text{James}}\mathit{\text{Webb}}\mathit{\text{Space}}\mathit{\text{Telescope}}$/Mid-Infrared Instrument Medium Resolution Spectrometer spectrum (4.9–27.9,$\text{upmu}$m) of the local ultra luminous infrared galaxy IRAS~07251$-$0248. We find that the molecular absorptions are blueshifted by 160,km,s${}^{-1}$ relative to the systemic velocity of the target. Using local thermal equilibrium (LTE) excitation models, we derive rotational temperatures ($T_{\mathrm{r}\mathrm{o}\mathrm{t}}$) from 42 to 185,K for these absorption bands. This range of measured $T_{\mathrm{r}\mathrm{o}\mathrm{t}}$ can be explained by infrared (IR) radiative pumping as a by–product of the strength, effective critical density, and opacity of each molecular band. Thus, these results suggest that these absorptions originate in a warm expanding gas shell ($\dot{M}$$\sim$90–330,$M_{\odot }$,yr${}^{-1}$), which might be the base of the larger scale cold molecular outflow detected in this source. Finally, the elevated abundance of molecular cations can be explained by a high cosmic ray ionization rate, with log(${\zeta }_{{\text{H}}_2}$/n${}_{\mathrm{H}}[{\text{cm}}^3{\text{s}}^{-1}])$ in the range of $-$18.2 (from H${}_3^{+}$) to $-$19.1 (inferred from HCO${}^{+}$ and N${}_2$H${}^{+}$, which are likely tracing denser gas), consistent with a cosmic ray dominated chemistry as predicted by chemical models.