Collisional growth of giant molecular clouds

The observation that giant molecular clouds in the inner galactic plane populate both arm and interarm regions has recently been interpreted as evidence of a very long lifetime (>108 years) for these clouds. An implication of this longevity is that the great masses of the clouds in the molecular ring at omega-tilde=4--8 kpc may be accumulated through cloud-cloud collisional accretion. Starting from an initial distribution of small clouds, the individual cloud masses will grow to the observed 105--106 M/sub sun/ within 109 years, assuming the total mass of H2 remains constant. Continued growth will be halted at a few times 105 M/sub sun/, where the collisional accretion rate is balanced by the removal of H2 into stars at a galactic rate of 2 M/sub sun/ Year-1 combined with ionization by H II regions