ON THE SIZE AND FORMATION MECHANISM OF STAR COMPLEXES IN SM, IM, AND BCD GALAXIES

The diameters D-c of the largest star-forming complexes in 67 Magellan ic spiral and irregular galaxies and 16 blue compact dwarf (BCD) galax ies are found to scale approximately with the square root of the galax y luminosity for each type, i.e., smaller galaxies have proportionatel y smaller star-forming regions. This is the same relation as for the l argest complexes in bright spiral galaxies found previously, although Sm/Im galaxies have complexes that, on average, are a factor of simila r to 2 larger than the extrapolation for spiral galaxies at the same a bsolute magnitude, and the BCD galaxies have complexes that are simila r to 2 times larger than those typical of the Sm/Im galaxies at the sa me absolute magnitude. These results are consistent with the interpret ation that the largest complexes form at the gravitational length scal e in a marginally stable interstellar medium with a nearly constant ve locity dispersion c similar to 5-10 km s(-1). The luminosity scaling i s then the result of higher average total densities in smaller galaxie s compared with the outer regions of giant spirals. This total density correlation is shown using published H I line widths and optical gala xy sizes. The implication of these results is that star formation begi ns when the ratio of the gas density rho to the total density (gas + s tars + dark matter) exceeds several tenths. If star formation lasts fo r a time scaling with (G rho)(-1/2) similar to D-c/c, then the main mo rphological differences between star formation in galaxies of various sizes can be explained: large galaxies have large star complexes that form groups of OB associations slowly for up to 50 Myr; small galaxies have small complexes (in terms of absolute size) that form dense asso ciations quickly, in bursts spanning less than 5 Myr.