AAS 201st Meeting, January, 2003
Session 127. Interstellar Medium: Hot and Cold
Oral, Thursday, January 9, 2003, 10:00-11:30am, 606-607

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[127.06] Why Galaxies Have Dust Lanes: A Sharp Transition in the Morphology of the Cold ISM

J. J. Dalcanton, P. Yoachim (UW)

Using a large sample of edge-on, bulgeless disk galaxies, we have identified a sharp mass-dependent transition in the structure of the cold dusty ISM. In more massive, rapidly rotating disks with Vrot>120{\rm\,km\,s-1}, we see the well defined dust lanes traditionally associated with edge-on galaxies. However, in more slowly rotating lower mass galaxies with Vrot<120{\rm\,km\,s-1}, we find {\emph{no}} dust lanes. The transition between these two states of the dusty ISM is extremely abrupt, with no scatter on either side of the Vrot=120{\rm\,km\,s-1} transition. We show that the transition is due primarily to the onset of disk instabilities. The galaxies with Vrot>120{\rm\,km\,s-1} are unstable to radial pertubations, as judged by the Qeff paramater for a mixed star+gas disk. In contrast, all of the galaxies Vrot<120{\rm\,km\,s-1} are stable. We argue that the disk instabilities are crucial for raising the density of the warm neutral medium to the point where it can cool efficiently into the cool neutral phase, which then condenses rapidly to the molecular phase. The cooling efficiency drops off dramatically above the plane, so that the cold ISM forms primarily in a thin layer, visible as a dust lane. In the absence of disk instabilities, only shocks and turbulence are capable of driving the densities high enough to cool to the molecular phase, leading to a filamentary morphology for the cold ISM. These two different modes of molecular gas formation lead to the two different dust morphologies that we observed in our sample. In the case of extremely high star formation rates, both modes can be present, leading to extraplanar dust filaments superimposed on dust lanes. We show that the transition in the morphology of the ISM leads to a fall-off in the molecular gas fraction as judged by the ratio of the CO(1-0) flux to the 21 cm line. However this sharp drop in the relative strength of the CO emission is accompanied by a sharp increase in the CO-to-H2 conversion factor X as well, such that the total molecular gas content probably changes less dramatically.

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