An interaction model for the formation of dwarf galaxies and 10⁸ M_⊙ clouds in spiral disks

Abstract

Galaxy interactions that agitate the interstellar medium by increasing the gas velocity dispersion and removing peripheral gas in tidal arms should lead to the formation and possible ejection of self-gravitationally bound cloud complexes with masses in excess of 108 M⊙. Some of these complexes may eventually appear as independent dwarf galaxies. The formation of clouds with masses exceeding 108 M⊙ is the result of gravitational instabilities in gas disks with high velocity dispersions. 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 >108 M⊙ and widespread velocity dispersions 4 times larger than in normal spiral galaxies. A giant cloud that forms by an instability in a high-dispersion ISM should also have a high internal dispersion, and it should produce stars with a greater efficiency than in normal galaxies because of the cloud's greater resistance to self-destruction. Such clouds should also have a larger fraction of massive stars than normal clouds because of the larger temperatures that follow from the high efficiency. Thus agitated galaxies should produce peripheral or nuclear starbursts partly because of their high gas velocity dispersions. Numerical N-body simulations of interacting galaxies illustrate the proposed formation of 108 M⊙ cloud complexes by gravitational instabilities. The masses and dispersions of the clouds that form increase with the strength of the perturbation. The simulations suggest that the complete detachment of an unbound dwarf galaxy requires a companion mass comparable to or larger than the galaxy mass. Dwarf galaxies that form this way should contain old stars from the original disk plus new stars from the cloud complex/starburst phase of its interaction-induced formation. The model also forms an extended gas pool containing 109 M⊙ at the end of the tidal arm opposite the companion. This low-density gas was uniformly distributed in the outer part of the disk before the interaction, and it too eventually leaves the galaxy.