ABSTRACT

Galaxy interactions that agitate the interstellar medium by increasing the gas velocity dispersion and removing peripheral gas in tidal arms can 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.

MASSIVE CLOUDS IN IC 2163/NGC 2207

VLA HI observations (Elmegreen et al. 1993a) reveal 10 clouds each with HI mass > 108 M⊙ in the outer parts and in the main disks of the interacting galaxy pair IC 2163/NGC 2207. Our observations apparently catch this pair in the early stages of massive cloud formation. The clouds, which are comparable in mass to dwarf galaxies, are fundamentally clumps in the gas, not clumps in the stellar component. The HI velocity dispersion in the clouds and in much of the main disk of NGC 2207 is, typically, 40 km s-1, a factor of 4 times higher than in normal disk galaxies. We propose that the high velocity dispersion of the gas is the key to why these clouds are at least 10 times more massive than the largest clouds in normal disk galaxies: the Jeans mass scales as the fourth power of the effective velocity dispersion. Such massive clouds can form by common gravitational instabilities where the Jeans mass is high and where the local value of the instability parameter for the gas is below threshold, e.g. in the outer disk and arms. Some of the massive clouds may later become large star formation complexes.