Oxygen Abundance Measurements of SHIELD Galaxies

Abstract

We have derived oxygen abundances for eight galaxies from the Survey for H I in Extremely Low-mass Dwarfs (SHIELD). The SHIELD survey is an ongoing study of very-low-mass galaxies, with M_H \scriptsize{I} between 10^6.5 and 10^7.5 M _⊙, that were detected by the Arecibo Legacy Fast ALFA survey. Hα images from the WIYN 3.5 m 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 4 m telescope at Kitt Peak National Observatory. We obtained a direct measurement of the electron temperature by detection of the weak [O III] λ4363 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.