Regular strings of HII regions and superclouds in spiral galaxies : clues to the origin of cloudy structure.

The large-scale dimensions of cloud and star formation are determined using studies of galaxies with long strings of regularly spaced, giant H II regions. The H II regions are found to be typically separated by distances equal to approximately 0.2 of the radii at 25 mag per sq arcsec surface brightness. Giant H I clouds in similar chains are found to show the same separations. These results indicate that coherent interstellar clouds may have dimensions as large as a kiloparsec or more, and that the individual clusters and OB associations formed by these clouds should group together into giant star complexes. The large scales observed for these features are determined to be consistent with their formation by a gravitational instability in the rotating, magnetic gas layers of galaxies. It is suggested that giant molecular clouds (M = 10 to the 5th solar masses) are only the dense and transient cores of larger H I clouds (M = 10 to the 7th solar masses), while individual molecular clouds are short lived, and the largest of these clouds should be in the spiral arms of galaxies.