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

Abstract: We analyze 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 JWST/MIRI MRS spectrum (4.9-27.9 $\mu$m) of the local ultra luminous infrared galaxy (ULIRG) 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 ($T{\rm rot}$) from 42 to 185 K for these absorption bands. This range of measured $T_{\rm rot}$ can be explained by infrared (IR) radiative pumping as a byproduct 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 (H$3^+$, HCO$^+$, HCNH$^+$, and N$_2$H$^+$) can be explained by a high cosmic ray ionization rate, with log($\zeta{\text{H}2}$/n${\rm H}\, [\text{cm}^3 \text{s}^{-1}])$ in the range of -19.1 to -18.2, consistent with a cosmic ray dominated chemistry as predicted by chemical models.