Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

## [45.10] Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81

R. J. Allen (STScI), J. Knapen (U. Montreal), R. Bohlin (STScI)

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.