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 galaxy, 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 from the plane on kpc scales. We interpret two HI loops, positioned at either end of the optical major axis, as expanding supershells. The eastern feature, in particular, has the clear signature of an expanding half-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 the most energetic HI supershell yet detected. Since this galaxy is isolated, the supershells are unlikely to have been created through impacting external clouds, yet the required input energy is also greater than 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 knowledge 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 profiles in the HI cube. The rotation curve, as illustrated by a major axis position velocity slice as well as first moment contours, appears to decline at radii > 14 kpc. (C) 1997 Elsevier Science B.V.