Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. HII region observations and chemical abundances

Abstract

In this paper we present the results of observations of seventeen HII regions in thirteen galaxies from the SIGRID sample of isolated gas rich irregular dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral [OIII] 4363A line, from which we calculate the electron temperature, Te, and gas-phase oxygen abundance. Five of the objects are blue compact dwarf (BCD) galaxies, of which four have not previously been analysed spectroscopically. We include one unusual galaxy which exhibits no evidence of the [NII] {\lambda}{\lambda} 6548,6584A lines, suggesting a particularly low metallicity (< Zsolar/30). We compare the electron temperature based abundances with those derived using eight of the new strong line diagnostics presented by Dopita et al. (2013). Using a method derived from first principles for calculating total oxygen abundance, we show that the discrepancy between the Te-based and strong line gas-phase abundances have now been reduced to within ~0.07 dex. The chemical abundances are consistent with what is expected from the luminosity-metallicity relation. We derive estimates of the electron densities and find them to be between ~5 and ~100 cm-3. We find no evidence for a nitrogen plateau for objects in this sample with metallicities 0.5 > Zsolar > 0.15.