Oxygen Abundance Measurements of SHIELD Galaxies

TL;DR

This study measures oxygen abundances in eight low-mass SHIELD galaxies, revealing insights into their metallicity and star formation, and suggesting a different mass-to-light ratio compared to typical dwarf galaxies.

Contribution

First direct oxygen abundance measurements for SHIELD galaxies, using both direct and strong-line methods, and analysis of their luminosity-metallicity relationship.

Findings

SHIELD galaxies show a shallower luminosity-metallicity slope.

Near-infrared luminosity reduces the offset in the metallicity relation.

Indications of differing star-formation histories in low-luminosity dwarfs.

Abstract

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 M$_{\rm HI}$ between 10$^{6.5}$ and 10$^{7.5}$ M$_{\odot}$, that were detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. H$\alpha$ 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] $\lambda$4363 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.