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

Galaxies with long strings of regularly spaced, giant H II regions are studied to determine the large-scale dimensions of cloud and star formation. The H II regions are typically separated by distances equal to ∼ 0.2 of the radii at 25 mag per arcsec² surface brightness, which correspond to spatial distances between condensations equal to 1–4 kpc for galaxies the size of our own. Giant H I clouds in similar chains show the same separations. These results imply 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. Gould's Belt and other examples of such complexes are discussed. The large scales commonly observed for these features are consistent with their formation by a gravitational instability in the rotating, magnetic gas layers of galaxies. Giant molecular clouds ($$(M=10^5M_\odot)$$) appear to be only the dense and transient cores of larger H I clouds ($$(M=10^7M_\odot$)$. Individual molecular clouds should be short lived, and the largest of these clouds should be in the spiral arms of galaxies.