2007ApJ...659..296L

High-resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C₂H₂, HCN, and CO₂molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n≳10⁷/cm3), warm (T_ex=200-700 K) molecular gas with high column densities of these molecules ranging from a few 10¹⁵ to 10¹⁷/cm2. Abundances relative to H₂, inferred from the silicate optical depth, range from ∼10^–7 to 10^–6 and show no correlation with temperature. Theoretical studies show that the high abundances of both C₂H₂and HCN exclude an X-ray dominated region (XDR) associated with the toroid surrounding an AGN as the origin of this dense warm molecular gas. Galactic massive protostars in the so-called hot-core phase have similar physical characteristics with comparable high abundances of C₂H₂, HCN, and CO₂in the hot phase. However, the abundances of C₂H₂and HCN and the C₂H₂/CO₂and HCN/CO₂ ratios are much higher toward the (U)LIRGs in the cooler (T_ex≲400 K) phase. We suggest that the warm dense molecular gas revealed by the mid-IR absorption lines is associated with a phase of deeply embedded star formation, where the extreme pressures and densities of the nuclear starburst environment have inhibited the expansion of H II regions and the global disruption of the star-forming molecular cloud cores and have ``trapped'' the star formation process in an ``extended'' hot-core phase.