Evidence for the Large‐Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81

We compare the detailed distributions of HI, H alpha, and 150 nm far-UVcontinuum emission in the spiral arms of M81 at a resolution of 9" (linearresolution 150 pc at 3.7 Mpc distance). The bright H alpha emission peaks arealways associated with peaks in the far-UV emission. The converse is not alwaystrue; there are many regions of far-UV emission with little corresponding Halpha. The HI and the far-UV are always closely associated, in the sense thatthe HI is often brightest around the edges of the far-UV emission. The effectsof extinction on the morphology are small, even in the far-UV. Extensive far-UVemission, often with little corresponding H alpha, indicates the presence ofmany ``B-stars'', which produce mostly non-ionizing UV photons. These far-UVphotons dissociate a small fraction of an extensive layer of H_2 into HI. Theobserved morphology can be understood if ``chimneys'' are common in the spiralarms of M81, where holes are blown out of the galactic disk, exposing thebright HII regions and the corresponding far-UV associated with vigorous starformation. These ``naked'' star-forming regions show little obscuration. H_2 isturned into HI by UV photons impinging on the interior surfaces of thesechimneys. The intensity of the far-UV radiation measured by UIT can dissociatethe underlying H_2 with a typical density of ~10 H nucleii cm**-3 to producethe observed amount of HI in the spiral arms of M81. Except for thin surfacelayers locally heated in these photo-dissociation regions close to the far-UVsources, the bulk of the molecular gas in the inner disk of M81 is apparentlytoo cold to produce much 12CO(1-0) emission.Comment: 12 pages, Latex. 8 postscript files. Better quality versions of the figures available from ftp://star.herts.ac.uk/pub/Knapen/m81uv . Accepted, Ap