Properties of H II region populations in galaxies. II - The H II region luminosity function

We have measured Hα emission-line fluxes for all of the detected H II regions in 30 nearby spiral and irregular galaxies, and have used these data to study the behavior of the H II region luminosity function (LF). In most galaxies the LF is well represented by a power-law function, with N (L) is proportional to L^-2+/-0. 5^. Both the normalization and the shape of the LF change systematically with Hubble type. Early-type spirals posses fewer H II regions of all luminosities, a steeper LF, and in many cases an upper cutoff or turnover in the LF. These effects taken together account for the virtual absence of "supergiant" H II regions such as 30 Doradus in Sa-Sb galaxies. Most of the massive stars in late-type spiral and irregular galaxies form in large H II/OB complexes, while in early-type spirals the bulk of the massive star formation occurs in small regions ionized by no more than a few OB stars. Within individual galaxies, the LF does not appear to change significantly with galactocentric radius, but we do observe a pronounced difference between the LF in spiral arm and interarm regions. The most straightforward explanation for the large changes in H II region populations along the Hubble sequence is a strong underlying gradient in the masses of interstellar atomic and molecular clouds.