Molecular Clouds in Starburst Galaxies
Starbursts in the centres of galaxies are believed to be caused by funneling of gas towards the inner regions. Gravitational interactions between galaxies are known to cause vast rearrangements of their structure as well as induce bursts of star formation in their nuclear regions. However, radial gas-transport may also be the result of a bar- instability intrinsic to the system. This report deals with the effect of a starburst on the molecular medium of galaxies. The mm-wave, rotational transitions of interstellar molecules serve as probes of the physical conditions, kinematics and total mass of the molecular gas. Observations of the J=1-0, J=2-1 and J=3-2 transitions of 12Co, the J=1-0 and J=2-1 transitions of 13Co, the J=1-0 transitions of C180 and HCN and the J=2-1 transition of CS are used to determine the properties of the molecular gas in starbursting and normal galaxies. The mean escape probability approximation is used to solve the radiative transfer equations.
The inferred properties of the molecular clouds in the centres of starburst galaxies differ from those of Galactic disk clouds. Such a deviation is likely to be induced by a central gas concentration and/or the nuclear starburst disrupting the clouds. Large 12CO/13CO J=1-0 intensity ratios (R>20) are measured towards luminous mergers implying unusual, high-pressure, molecular cloud ensembles.
Model cloud ensembles for the merging system, NGC 3256, and the starburst spiral NGC 1808 suggest that the 12CO-emission is dominated by warm (Tk = 100 - 300 K), small (0.5-1 pc) and moderately dense (n = 2 - 7 x 103 cm-3) molecular clouds. Such temperatures and densities are typical of Galactic photon dominated regions (PDRs). Statistical studies of molecular line ratios towards galaxy centres indicate that the optical depth of the 12CO 1-0 emission is moderate, .tau.Ã? 1. Many starburst galaxies have large-scale excitation gradients indicating warm, dense gas in the centre, and normal cool molecular gas in their extended disks. NGC 3256, in particular, has a large R = 35 in the centre, while it is found to be only R = 10 - 15 at positions 43" off centre. The model cloud ensemble of NGC 1808 indicate that the 12CO-emitting gas may be diffuse rather than self-gravitating, this could be a common phenomenon in starburst galaxies.