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

We study the 3.4 − 4.4 μm fundamental rovibrational band of H 3 + , a key tracer of the ionization of the molecular interstellar medium (ISM), in a sample of 12 local ( d < 400 Mpc) (ultra)luminous infrared galaxies ((U)LIRGs) observed with JWST/NIRSpec. The P , Q , and R branches of the band are detected in 13 out of 20 analyzed regions within these (U)LIRGs, which increases the number of extragalactic H 3 + detections by a factor of 6. For the first time in the ISM, the H 3 + band is observed in emission; we detect this emission in three regions. In the remaining ten regions, the band is seen in absorption. The absorptions are produced toward the 3.4 − 4.4 μm hot dust continuum rather than toward the stellar continuum, indicating that they likely originate in clouds associated with the dust continuum source. The H 3 + band is undetected in Seyfert-like (U)LIRGs where the mildly obscured X-ray radiation from the active galactic nuclei might limit the abundance of this molecule. For the detections, the H 3 + abundances, N (H 3 + )/ N H = (0.5 − 5.5)×10 −7 , imply relatively high ionization rates, ζ H 2 , of between 3 × 10 −16 and > 4 × 10 −15 s −1 , which are likely associated with high-energy cosmic rays. In half of the targets, the absorptions are blueshifted by 50–180 km s −1 , which is lower than the molecular outflow velocities measured using other tracers such as OH 119 μm or rotational CO lines. This suggests that H 3 + 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 seven out of ten objects, the H 3 + excitation is consistent with inelastic collisions with H 2 in warm translucent molecular clouds ( T kin ∼ 250–500 K and n (H 2 ) ∼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 H 3 + detected for the first time in the ISM.