The intergalactic HI cloud in Leo : a simple modeling of the Spitzer-Baade collision event.

Evidence is presented to test the hypothesis that the intergalactic H I cloud in Leo is a nearly freely expanding product of a Spitzer-Baade (gas-sweeping) collision between two galaxies (NGC 3368 and NGC 3384) in GWa. A controlled experiment identifies the two colliders. The galaxies are identified by an a priori selection criterion among a well-defined sample of seven galaxies and lie at equal angular distances on either side of the center of the cloud. Traces of two optical rings in NGC 3368 and a spiral arm in NGC 3384 are observed. These features are found in computer-simulated collisions by Theys and Spiegel (1977) and Toomre (1978). A one-arm filament in NGC 3368 which extends in the direction of the H I cloud corresponds to features expected from gas-sweeping and tidal interactions of colliding galaxies. Based on observational data, a simple physical model of the gas-sweeping collision event is constructed.