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

April 2007 • 2007ApJ...659..296L

Authors • Lahuis, F. • Spoon, H. W. W. • Tielens, A. G. G. M. • Doty, S. D. • Armus, L. • Charmandaris, V. • Houck, J. R. • Stäuber, P. • van Dishoeck, E. F.

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>~10⁷ cm^-3), warm (T_ex=200-700 K) molecular gas with high column densities of these molecules ranging from a few 10¹⁵ to 10¹⁷ cm^-2. 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 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, HCN, and CO₂ in the hot phase. However, the abundances of C₂H₂ and HCN and the C₂H₂/CO₂ and 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.

Links

View on ADS

IPAC Authors
(alphabetical)

Lee Armus

Senior Scientist