Session 68 - Disk Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,

## [68.05] Neutral Hydrogen in M 108 (NGC 3556) and the Discovery of HI Supershells

D. L. Giguere, J. Irwin (Queen's U.)

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. Two HI arcs, positioned at either end of the optical major axis have the signature of expanding shells and, in the context of energy input from supernovae and stellar winds, the required input energy for the eastern shell is > 2.6 \times 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 observed internal star formation rate. Thus it would appear that some form of energy enhancement (such as magnetic fields) must also 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 mistakenly 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 appears to decline at radii > 14 kpc.