High resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C _ { 2 } H _ { 2 } , HCN , and CO _ { 2 } molecules toward a sample of deeply obscured ( U ) LIRG nuclei .
The observed bands reveal the presence of dense ( n \gtrsim 10 ^ { 7 } \mathrm { cm } ^ { -3 } ) , warm ( T _ { \mathrm { ex } } = 200 - 700 \mathrm { K } ) molecular gas with high column densities of these molecules ranging from a few 10 ^ { 15 } -10 ^ { 17 } \mathrm { cm } ^ { -2 } .
Abundances relative to H _ { 2 } , inferred from the silicate optical depth , range from \sim 10 ^ { -7 } to 10 ^ { -6 } and show no correlation with temperature .
Theoretical studies show that the high abundances of both C _ { 2 } H _ { 2 } and HCN exclude a 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 C _ { 2 } H _ { 2 } , HCN , and CO _ { 2 } in the hot phase .
However , the abundances of C _ { 2 } H _ { 2 } and HCN and the C _ { 2 } H _ { 2 } /CO _ { 2 } and HCN/CO _ { 2 } ratios are much higher toward the ( U ) LIRGs in the cooler ( T _ { \mathrm { ex } } \lesssim 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 ‘ trapped ’ the star formation process in an ‘ extended ’ Hot Core phase .