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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. TheP,Q, andRbranches 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 × 10⁻¹⁶and > 4 × 10⁻¹⁵s⁻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 H₂in warm translucent molecular clouds (T_kin ∼ 250–500 K andn(H₂) ∼10^2 − 3cm⁻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. 

Understanding the nature of high-redshift dusty galaxies requires a comprehensive view of their interstellar medium (ISM) and molecular complexity. However, the molecular ISM at high redshifts is commonly studied using only a few species beyond¹²C¹⁶O, limiting our understanding. In this paper, we present the results of deep 3 mm spectral line surveys using the NOrthern Extended Millimeter Array (NOEMA) targeting two strongly lensed dusty galaxies observed when the Universe was less than 1.8 Gyr old: APM 08279+5255, a quasar at redshiftz= 3.911, and NCv1.143 (H-ATLAS J125632.7+233625), az= 3.565 starburst galaxy. The spectral line surveys cover rest-frame frequencies from about 330 to 550 GHz for both galaxies. We report the detection of 38 and 25 emission lines in APM 08279+5255 and NCv1.143, respectively. These lines originate from 17 species, namely CO,¹³CO, C¹⁸O, CN, CCH, HCN, HCO⁺, HNC, CS, C³⁴S, H₂O, H₃O⁺, NO, N₂H⁺, CH, c-C₃H₂, and the vibrationally excited HCN and neutral carbon. The spectra reveal the chemical richness and the complexity of the physical properties of the ISM. By comparing the spectra of the two sources and combining the analysis of the molecular gas excitation, we find that the physical properties and the chemical imprints of the ISM are different: the molecular gas is more excited in APM 08279+5255, which exhibits higher molecular gas temperatures and densities compared to NCv1.143; the molecular abundances in APM 08279+5255 are akin to the values of local active galactic nuclei (AGN), showing boosted relative abundances of the dense gas tracers that might be related to high-temperature chemistry and/or the X-ray-dominated regions, while NCv1.143 more closely resembles local starburst galaxies. The most significant differences between the two sources are found in H₂O: the 448 GHz ortho-H₂O(4₂₃ − 3₃₀) line is significantly brighter in APM 08279+5255, which is likely linked to the intense far-infrared radiation from the dust powered by AGN. Our astrochemical model suggests that, at such high column densities, far-ultraviolet radiation is less important in regulating the ISM, while cosmic rays (and/or X-rays and shocks) are the key players in shaping the molecular abundances and the initial conditions of star formation. Both our observed CO isotopologs line ratios and the derived extreme ISM conditions (high gas temperatures, densities, and cosmic-ray ionization rates) suggest the presence of a top-heavy stellar initial mass function. From the ∼330–550 GHz continuum, we also find evidence of nonthermal millimeter flux excess in APM 08279+5255 that might be related to the central supermassive black hole. Such deep spectral line surveys open a new window into the physics and chemistry of the ISM and the radiation field of galaxies in the early Universe. 

We report molecular gas observations of IRAS 20100-4156 and IRAS 03158+4227, two local ultraluminous infrared galaxies (ULIRGs) hosting some of the fastest and most massive molecular outflows known. Using ALMA and PdBI observations, we spatially resolve the CO(1-0) emission from the outflowing molecular gas in both and find maximum outflow velocities of $$ v_{\rm max} \sim 1600$$ and $$\sim 1700$$ km/s for IRAS 20100-4156 and IRAS 03158+4227, respectively. We find total gas mass outflow rates of $$\dot M_{\rm OF} \sim 670$$ and $$\sim 350$$ Msun/yr, respectively, corresponding to molecular gas depletion timescales $$\tau^{\rm dep}_{\rm OF} \sim 11$$ and $$\sim 16$$ Myr. This is nearly 3 times shorter than the depletion timescales implied by star formation, $$\tau^{\rm dep}_{\rm SFR} \sim 33$$ and $$\sim 46$$ Myr, respectively. To determine the outflow driving mechanism, we compare the starburst ($$L_{*}$$) and AGN ($$L_{\rm AGN}$$) luminosities to the outflowing energy and momentum fluxes, using mid-infrared spectral decomposition to discern $$L_{\rm AGN}$$. Comparison to other molecular outflows in ULIRGs reveals that outflow properties correlate similarly with $$L_{*}$$ and $$L_{\rm IR}$$ as with $$L_{\rm AGN}$$, indicating that AGN luminosity alone may not be a good tracer of feedback strength and that a combination of AGN and starburst activity may be driving the most powerful molecular outflows. We also detect the OH 1.667 GHz maser line from both sources and demonstrate its utility in detecting molecular outflows.