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

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Abstract
High-resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C2H2, HCN, and CO2 molecules toward a sample of deeply obscured (U)LIRG nuclei. The observed bands reveal the presence of dense (n k107 cm� 3), warm (Tex ¼ 200Y700 K) molecular gas with high column densities of these moleculesrangingfromafew1015to1017cm� 2.AbundancesrelativetoH2,inferredfromthesilicateopticaldepth, rangefrom � 10� 7to10� 6andshownocorrelationwithtemperature.Theoreticalstudiesshowthatthehighabundances of both C2H2 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 C2H2 ,H CN, and CO2 in the hot phase. However, the abundances of C2H2and HCN and the C2H2/CO2 and HCN/CO2 ratios are much higher toward the (U)LIRGs in the cooler (Tex P 400K) phase. We suggest that the warm dense moleculargas 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. Subject headinggs: galaxies: ISM — galaxies: nuclei — infrared: ISM — ISM: evolution — ISM: molecules

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