Radio Jet-blown Neutral Hydrogen Supershells in Spiral Galaxies?
  Taking a clue from the pair of HI supershells found in the Scd galaxy NGC 3556 (M 108), we propose a new mechanism for the origin of HI supershells in gas-rich massive galaxies. In this scenario, the two supershells were inflated out of the neutral hydrogen disk due to the localised flaring of a pair of radio lobes formed by the jets ejected from the nucleus during an active phase about 10^7 years ago, but have faded away by now. It is shown that the salient features of this supershell pair, such as their symmetrical locations about the galactic centre, the anomalously large energy requirements, the large galactocentric distances, as well as the Z-symmetric hemispherical shapes, find a more natural explanation in terms of this scenario, as compared to the standard models which postulate either a massive starburst, or the infall of external gas clouds. Other possible implications of this hypothesis are briefly discussed. 
Gopal-Krishna and Judith A. Irwin