UBVR and H alpha photometry of H II regions and OB associations in galaxies: A test for a variable IMF

Photometry in UBVR and H alpha of H II regions and OB associations in a sample of Sa-Scd galaxies is presented. The distributions of H alpha equivalent widths for H II regions show no significant dependence on morphological type, and the continuum luminosity functions of the OB associations closely parallel the H alpha luminosity functions of the H II regions. Both luminosity functions show a strong dependence on Hubble type, in the sense that the characteristic luminosity of the brightest H II regions/OB associations decreases sharply in early-type disks. This implies that the changes in H II region and association properties along the Hubble sequence are most likely due to changes in the clustering properties of massive stars, rather than in the mass function. (C) 1997 American Astronomical Society.