We have derived oxygen abundances for 8 galaxies from the Survey of HI in Extremely Low-mass Dwarfs (SHIELD). The SHIELD survey is an ongoing study of very low-mass galaxies, with MHI between 106.5 and 107.5 M⊙, that were detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. Hα images from the WIYN 3.5m telescope show that these 8 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 KPNO. We obtained a direct measurement of the electron temperature by detection of the weak [O III] λ4363 line in 2 of the HII regions. Oxygen abundances for the other HII regions were estimated using a strong-line method. When the SHIELD galaxies are plotted on a B-band luminosity-metallicity 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 this may be indicative of differing star-formation histories in the lowest luminosity and surface brightness dwarf irregulars.
@article{arxiv.1501.05159,
title = {Oxygen Abundance Measurements of SHIELD Galaxies},
author = {Nathalie C. Haurberg and John J. Salzer and John M. Cannon and Melissa V. Marshall},
journal= {arXiv preprint arXiv:1501.05159},
year = {2015}
}