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| Paper: |
Properties of the Dense Interstellar Medium in the Central Regions of Normal and Starburst Galaxies |
| Volume: |
275, Disks of Galaxies: Kinematics, Dynamics and Perturbations |
| Page: |
243 |
| Authors: |
Hüttemeister, S. |
| Abstract: |
The physical properties of the dense component of the Interstellar Medium (ISM), i.e. the molecular gas, in the central regions of galaxies are significantly different from those in more quiescent disk regions, like spiral arms. The dense ISM in the central part of a galaxy is, on average, both warmer and denser than in the disk, but in detail shows a complex multi-component structure. This may include `diffuse' unbound gas and dense, warm star-forming cores, as well as cool, dense cores, depending on whether the central region in question is engaged in a starburst or (relatively) less active. Even the seemingly most basic property of the molecular gas, its mass, cannot be determined in a straightforward way. Multilevel, multispecies studies of molecular lines, preferentially at high spatial resolution, are a powerful diagnostic to derive the structure and physical properties of the gas. A special role falls to the ratio of the CO isotopomers 12CO and 13CO, but high-density tracers like HCN, HNC and CN are crucial in showing that not even all starburst regions are identical. Using these diagnostics, the changes in gas properties can be traced through a bar (a frequent ingredient when gas is concentrated to the center) to the nuclear region. This contribution examines which gas components dominate in a number of different types of environment and characterizes their properties in simple scenarios. |
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