2024A&A...689L..12P

We study the 3.4 - 4.4 μm fundamental rovibrational band of H₃⁺, 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 H₃⁺ detections by a factor of 6. For the first time in the ISM, the H₃⁺ 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 H₃⁺ 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 H₃⁺ abundances, N(H₃⁺)/N_H = (0.5 - 5.5)×10^–7, imply relatively high ionization rates, ζ_H2, 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 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 seven out of ten objects, the H₃⁺ excitation is consistent with inelastic collisions with H₂ in warm translucent molecular clouds (T_kin ∼ 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.