Infrared Molecular Starburst Fingerprints in Deeply Obscured (Ultra)Luminous Infrared Galaxy Nuclei

Abstract

High-resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C_2H_2, HCN, and CO_2 molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n ≳ 10^7 cm^(-3)), warm (T_(ex) = 200-700 K) molecular gas with high column densities of these molecules ranging from a few 10^(15) to 10^(17) cm^(-2). Abundances relative to H_2, 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 C_2H_2 and 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 C_2H_2, HCN, and CO_2 in the hot phase. However, the abundances of C_2H_2 and HCN and the C_2H_2/CO_2 and HCN/CO_2 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.