Abstract
A program of direct photography and spectrophotometry has been carried out on H ii regions in a selection of nearby spiral and irregular galaxies. Abundance analyses for H ii regions in M101 and M33 show a strong radial gradient in the O/H ratio, decreasing by approximately a factor of 10 across the galaxy disks. The Ne/O and S/O abundances ratios are constant. The derived nitrogen abundances, coupled with the line ratios for other H ii regions, indicate a weak gradient in the ratio of N/O, decreasing by about a factor of 4 from the nuclear regions outward. No large helium abundance differences are observed, although the most oxgyen-poor regions may show marginally significant helium deficiencies. Excitation differences among the H ii regions of Sbc-Scd-Irr galaxies can best be understood in terms of an abundance sequence which progresses from higher to lower heavy-element enrichment as one progresses from the earlier to later type galaxies.
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