AAS 207th Meeting, 8-12 January 2006
Session 134 Irregular Galaxies
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[134.03] Oxygen and Nitrogen Enrichment in Dwarf Irregular Galaxies

L. van Zee (Indiana University)

Results of long slit optical spectroscopy of HII regions in 40 dwarf irregular galaxies are considered in the context of closed and open box models of chemical evolution. While the observed oxygen abundances for several galaxies are consistent with closed box chemical evolution, the majority of this sample have an effective yield ~1/4 of the expected yield for a constant star formation rate and Salpeter IMF, indicating that either outflow of enriched gas or inflow of pristine gas has occurred. The effective yield strongly correlates with MH/LB in the sense that gas-rich galaxies are more likely to be closed systems. However, the effective yield does not appear to correlate with other global parameters such as dynamical mass, absolute magnitude, star formation rate or surface brightness. In addition, open and closed systems are not identified easily in other global abundance measures; for example, the observed correlation between luminosity and metallicity is consistent with other recent results in the literature. A correlation is found between the observed nitrogen--to--oxygen ratio and the color of the underlying stellar population; redder dwarf irregular galaxies have higher N/O ratios than blue dwarf irregular galaxies. The relative abundance ratios are interpreted in the context of delayed release of nitrogen and varied star formation histories.

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