Structure and evolution of galaxy disks

Using the Palomar 5m telescope, we have measured the abundances in 185 HII regions spanning a range of radius in 13 spiral galaxies. As expected, the nitrogen-to-oxygen ratio increases linearly with the oxygen abundance for high metallicity HII regions, indicating that nitrogen is predominantly a secondary element. However, the nitrogen to oxgen ratio plateaus for oxygen abundances less than 1/3 of solar, as is also seen in low metallicity dwarf galaxies. This result suggests that the observed trend in dwarf galaxies is not due to outflow of enriched material in a shallow gravitational potential.

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