ON THE SIZE AND FORMATION MECHANISM OF THE LARGEST STAR-FORMING COMPLEXES IN SPIRAL AND IRREGULAR GALAXIES

The average diameters of the largest star complexes in most of the spiral and irregular galaxies in the Sandage and Bedke Atlas of Galaxies were measured from the Atlas photographs. The complex diameters D(c) correlate with galaxy magnitude as D(c) = 0.18 - 0.14M(B), which has about the same slope as the correlation for the largest H II regions studied by Kennicutt. There is no obvious correlation between D(c) and either Hubble type or spiral arm class at a given magnitude. The variation of D(c) with M(B) closely matches the expected variation in the characteristic length of the gaseous gravitational instability considering that the rotation curve varies with M(B) and that the stability parameter Q is about 1 in the outer regions of the disk. This match corresponds to an effective velocity dispersion of 6.1 km s-1 that is about the same for all spiral and irregular galaxies.