A GRAVITATIONAL GALACTIC WAKE IN THE M 81 GROUP

Recent H I observations of the M 81 group made with the VLA (Yun 1992; Yun et al. 1993) have revealed a previously unknown structure: a long ''bridge'' of gas stretching from NGC 3077 towards north-west. In a p revious paper (Thomasson & Donner 1993), we presented a model of the t idal interaction between M 81 and NGC 3077, aimed at explaining the sp iral pattern in M 81 and the long known bridge of gas between the two galaxies. We now show that if our previous model is supplemented by an additional large disc of gas belonging to M 81 (in agreement with Yun et al.'s observations), a narrow gaseous feature forms in the gravita tional wake behind NGC 3077. This feature closely resembles the bridge observed by Yun et al. Our model of the bridge, as opposed to e.g. ti dal or ram pressure stripping models, does not require that NGC 3077 o nce possessed a gas disc.