The chemistry of galaxies. I. The nature of giant extragalactic H II regions.

The emission properties presently derived from spectrophotometric observations of 99 H II regions in 20 spiral and irregular galaxies form a one-parameter sequence, and the oxygen abundance is found to be the underlying physical determinant of the spectrum of a giant extragalactic H II region. The ionizing radiation field hardens with decreasing abundance, and oxygen and nitrogen abundances appear to be closely coupled above an oxygen abundance of about 0.2 times that of the sun. Sulfur appears to grow as a primary product of nucleosynthesis with respect to oxygen. The visual continuum of the H II regions seems to be primarily contributed by nonionizing supergiants in the embedded OB associations. The bulk of the extinction of the nebular emission is caused by surrounding dust clouds and dust uniformly distributed throughout the disk of the host galaxy.