AN INTERACTION-MODEL FOR THE FORMATION OF DWARF GALAXIES AND 10(8) M-CIRCLE-DOT CLOUDS IN SPIRAL DISKS

Galaxy interactions that agitate the interstellar medium by increasing the ps velocity dispersion and removing peripheral gas in tidal arms should lead to the formation and possible ejection of self-gravitation ally bound cloud complexes with masses in excess of 10(8) M . . Some o f these complexes may eventually appear as independent dwarf galaxies. The formation of clouds with masses exceeding 10(8) M . is the result of gravitational instabilities in gas disks with high velocity disper sions. Such masses and high dispersions were observed with the VLA for the interacting pair IC 2163/NGC 2207, which contains 10 clouds with H I masses > 10(8) M . and widespread velocity dispersions 4 times lar ger than in normal spiral galaxies. A giant cloud that forms by an ins tability in a high-dispersion ISM should also have a high internal dis persion, and it should produce stars with a greater efficiency than in normal galaxies because of the cloud's greater resistance to self-des truction. Such clouds should also have a larger fraction of massive st ars than normal clouds because of the larger temperatures that follow from the high efficiency. Thus agitated galaxies should produce periph eral or nuclear starbursts partly because of their high gas velocity d ispersions. Numerical N-body simulations of interacting galaxies illus trate the proposed formation of 10(8) M . cloud complexes by gravitati onal instabilities. The masses and dispersions of the clouds that form increase with the strength of the perturbation. The simulations sugge st that the complete detachment of an unbound dwarf galaxy requires a companion mass comparable to or larger than the galaxy mass. Dwarf gal axies that form this way should contain old stars from the original di sk plus new stars from the cloud complex/starburst phase of its intera ction-induced formation. The model also forms an extended gas pool con taining 10(9) M . at the end of the tidal arm opposite the companion. This low-density gas was uniformly distributed in the outer part of th e. disk before the interaction, and it too eventually leaves the galax y.