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 Magellanic spiral and irregular galaxies and 16 blue compact dwarf (BCD) galaxies are found to scale approximately with the square root of the galaxy luminosity for each type, i.e., smaller galaxies have proportionately smaller star-forming regions. This is the same relation as for the largest complexes in bright spiral galaxies found previously, although Sm/Im galaxies have complexes that, on average, are a factor of similar to 2 larger than the extrapolation for spiral galaxies at the same absolute magnitude, and the BCD galaxies have complexes that are similar to 2 times larger than those typical of the Sm/Im galaxies at the same absolute magnitude. These results are consistent with the interpretation that the largest complexes form at the gravitational length scale in a marginally stable interstellar medium with a nearly constant velocity dispersion c similar to 5-10 km s(-1). The luminosity scaling is then the result of higher average total densities in smaller galaxies compared with the outer regions of giant spirals. This total density correlation is shown using published H I line widths and optical galaxy sizes. The implication of these results is that star formation begins when the ratio of the gas density rho to the total density (gas + stars + dark matter) exceeds several tenths. If star formation lasts for a time scaling with (G rho)(-1/2) similar to D-c/c, then the main morphological 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 associations quickly, in bursts spanning less than 5 Myr.