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Paper: The Way to the Center: Molecular Gas in Bars
Volume: 320, The Neutral ISM in Starburst Galaxies
Page: 117
Authors: Huttemeister, A.
Abstract: Bars are the most obvious mechanism by which gas is fed into the central regions of galaxies, where it may then trigger starburst activity. Only a minority of bars are actively funnelling gas toward the nucleus and can thus be traced by dense, molecular gas. These objects, however, offer an excellent laboratory for the study of changing gas properties, diagnosed by specific ratios between molecular lines. Ratios of CO isotopomer lines are of special importance and allow to identify a 'diffuse', unbound molecular phase as well as bound, almost disk-like clouds which may be the sites of star formation within bars. The diffuse medium which may dominate the region of the strongest bar shock is capable of offsetting the standard molecular mass determination from CO intensities. The examination of a small sample of gas-filled bars shows that not all specimens are equal — the shock strength and probably also the evolutionary stage of the (starburst) nuclei differ significantly. While it seems obvious that the fraction of dense gas rises if and when it becomes involved in starburst, a few starbursts are found where very faint HCN emission indicates that the amount of dense gas may be surprisingly low. In the future, thermal dust emission should represent another window into the varying properties of gas within bars.
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