NEUTRAL HYDROGEN IN M-108 (NGC-3556) AND THE DISCOVERY OF HI SUPERSHELLS

We present the first high resolution HI maps of the nearby, edge-on ga laxy, M 108 (NGC 3556). This galaxy is known to have a radio continuum thick disk and we have now found HI arcs and extensions protruding fr om the plane on kpc scales. We interpret two HI loops, positioned at e ither end of the optical major axis, as expanding supershells. The eas tern feature, in particular, has the clear signature of an expanding h alf-shell and, in the context of an origin from supernovae and stellar winds, the required input energy is > 2.6 x 10(56) erg, making this t he most energetic HI supershell yet detected. Since this galaxy is iso lated, the supershells are unlikely to have been created through impac ting external clouds, yet the required input energy is also greater th an that available from the currently observed internal star formation rate, Thus some additional energy enhancement (such as magnetic fields ) appears to be important in creating these features. The supershells are so dominant that they distort the outer major axis. Without a know ledge of the resolved structure of these features, the galaxy would be considered warped. We have also modeled the underlying smooth density and velocity distributions of this galaxy by reproducing the line pro files in the HI cube. The rotation curve, as illustrated by a major ax is position velocity slice as well as first moment contours, appears t o decline at radii > 14 kpc. (C) 1997 Elsevier Science B.V.