7 Conclusions

 

Optical emission-line spectroscopy of H II regions was obtained in dwarf irregular galaxies in the Centaurus A group, the Sculptor group, the Antlia-Sextans group, the Local Group, and in the field. [O III] $\lambda 4363$ was measured in A1346-358, IC 1613, and IC 5152, and the standard method was used to obtain oxygen abundances. For the remaining galaxies, the bright-line method was used to compute oxygen abundances with the McGaugh and Pilyugin calibrations. The [N II]/[O II] intensity ratio was used to break the degeneracy in the bright-line method and applied to both calibrations. For NGC 3109 and NGC 5264, the [N II]/H$\alpha$ intensity ratio was also used to confirm their abundances. ESO358-G030 has the lowest oxygen abundance in the sample with a value of 12+log(O/H) = 7.32, which is comparable to the value for the second most metal-poor galaxy known (SBS 0335-052).

Oxygen abundances for dwarfs in the southern sample agree with the metallicity-luminosity relationship defined by dwarf irregulars in the control sample. Three dwarfs with direct abundances are found to be in very good agreement with the relation, whereas the remaining galaxies with bright-line abundances are in comparable agreement with larger scatter. NGC 5264 appears to have an oxygen abundance approximately two times larger than expected for its luminosity. Extensive spectroscopy of H II regions in NGC 5264 will confirm whether the galaxy is a dwarf irregular galaxy with a zero or very small abundance gradient or a low-luminosity spiral galaxy with a significant abundance gradient.

Nitrogen-to-oxygen abundance ratios for dwarfs in the southern sample are comparable to those in the control sample at a given oxygen abundance, although IC 1613 and IC 5152 have larger N/O values compared to nearby dwarf irregulars at a given oxygen abundance. Additional stellar photometry for IC 5152 should provide further clues to the star formation and enrichment history of this nearby dwarf.

Acknowledgements

We are grateful to the anonymous referee for improving the presentation of the manuscript, and to the CTIO staff for their help with the observations. Partial support for this work was provided by NASA through grant GO-08192.97A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. H. L. acknowledges MPIA for their financial support, and Stephanie Côté and Evan Skillman for discussions and making available a copy of their paper before publication. Some data were accessed as Guest User, Canadian Astronomy Data Center, which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada's Herzberg Institute of Astrophysics. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.