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Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We compare the details of the distributions of HI, H\alpha, and far-UV emission in the spiral arms of M81, at a resolution of about 9'' (linear resolution 150 pc at 3.7 Mpc distance). The effects of extinction on the morphology of the H\alpha and far-UV are shown to be essentially bi-modal, i.e.\ the extinction is either very small or very large. The observed structures can be understood if ``chimneys'' are common in the spiral arms of M81, where holes are blown out of the galactic disk by star-formation activity similar to that discovered above the Galactic HII region IC1805 by Dewdney and his collaborators at DRAO. We propose further that H_2 is turned into HI on a large scale by UV photons impinging on the (rough) surfaces of these chimneys, analogous to the situation proposed to explain the morphology of the Galactic star-forming region G216-2.5 by Williams amp; Maddalena. In this picture, most of the atomic hydrogen in the inner spiral arms of M81 is UV-dissociated molecular hydrogen from an underlying (and as yet largely undetected) reservoir of molecular gas.
