Star-Forming Complexes in Galaxies

Star complexes are the largest globular regions of star formation in galaxies. If there is a spiral density wave, nuclear ring, tidal arm, or other well-defined stellar structure, then gravitational instabilities in the gaseous component produce giant cloud complexes with a spacing of about three times the width. These gas complexes form star complexes, giving the familiar beads on a string of star formation along spiral arms, or nuclear hotspots in the case of a ring. Turbulence compression, supernovae, and self-gravitational contraction inside the giant clouds produce a nearly scale-free structure, including giant molecular clouds that form OB associations and molecular cloud cores that form clusters. Without stellar density waves or similar structures, random gravitational instabilities form flocculent spirals and these fragment into star complexes, OB associations and star clusters in the same way. The largest coherent star-forming regions are the flocculent arms themselves. At the core of the hierarchy are the very dense clumps in which individual and binary stars form. The overall star formation rate in a galaxy appears to be regulated by gravitational collapse on large scales, giving the Kennicutt/Schmidt law scaling with density, but the efficiency factor in front of this scaling law depends on the fraction of the gas that is in a dense form. Turbulence compression probably contributes to this fraction. The CO version of the Schmidt law, recently derived by Heyer et al. (2004), follows from the turbulent hierarchy as well, as do the local efficiencies of star formation in OB associations and clusters. The efficiency of star formation increases with cloud density, and this is why most stars form in clusters that are initially self-bound.