H₃^+ absorption and emission in local U/LIRGs with JWST/NIRSpec: Evidence for high H₂ ionization rates

We study the 3. 4-4. 4m fundamental rovibrational band of H3+, a key tracer of the ionization of the molecular interstellar medium (ISM), in a sample of 12 local (d4x10^-15 s^-1, which are likely associated with high-energy cosmic rays. In half of the targets the absorptions are blue-shifted by 50-180 km/s, which are lower than the molecular outflow velocities measured using other tracers such as OH 119m or rotational CO lines. This suggests that H3+ traces gas close to the outflow launching sites before it has been fully accelerated. We used nonlocal thermodynamic equilibrium models to investigate the physical conditions of these clouds. In 7 out of 10 objects, the H3+ excitation is consistent with inelastic collisions with H2 in warm translucent molecular clouds (Tₖin ~ 250-500 K and n (H2) ~ 10^ (2-3) cm^-3). In three objects, dominant infrared pumping excitation is required to explain the absorptions from the (3, 0) and (2, 1) levels of H3+ detected for the first time in the ISM.