OXYGEN ABUNDANCE MEASUREMENTS OF SHIELD GALAXIES

We have derived oxygen abundances for eight galaxies from the Survey for Hi in Extremely Low-mass Dwarfs (SHIELD). The SHIELD survey is an ongoing study of very-low-mass galaxies, with M-HI between 10(6.5) and 10(7.5)M(circle dot), that were detected by the Arecibo Legacy Fast ALFA survey. Ha images from the WIYN 3.5m telescope show that these eight SHIELD galaxies each possess one or two active star-forming regions, which were targeted with long-slit spectral observations using the Mayall 4m telescope at Kitt Peak National Observatory. We obtained a direct measurement of the electron temperature by detection of the weak [O III] lambda 4363 line in two of the H II regions. Oxygen abundances for the other H II regions were estimated using a strong-line method. When the SHIELD galaxies are plotted on a B-band L-Z diagram, they appear to suggest a slightly shallower slope to the relationship than normally seen. However, that offset is systematically reduced when the near-infrared luminosity is used instead. This indicates a different mass-to-light ratio for the galaxies in this sample and we suggest that this may be indicative of differing star formation histories in the lowest luminosity and surface brightness dwarf irregulars.