Radio jet-blown neutral hydrogen supershells in spiral galaxies?

Taking a clue from the pair of HI supershells found in the Scd galaxy NGC 3 556 (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 infl ated out of the neutral hydrogen disk due to the localised flaring of a pai r of radio lobes formed by the jets ejected from the nucleus during an acti ve phase about similar to 10(7) years ago, but have faded away by now. It i s shown that the salient features of this supershell pair, such as their sy mmetrical locations about the galactic centre, the anomalously large energy requirements, the large galacto-centric distances, as well as the Z-symmet ric hemispherical shapes, find a more natural explanation in terms of this scenario, as compared to the standard models which postulate either a massi ve starburst. or the infall of external gas clouds. Other possible implicat ions of this hypothesis are briefly discussed.