Nebular metallicities in isolated dwarf irregular galaxies

Abstract

The motive for this work was to investigate whether small, isolated gas-rich galaxies show evidence of chemical evolution, by studying their nebular metallicities. I have identified a sample of 83 objects chosen for low luminosity and mass, the presence of active star formation, and isolation from other galaxies and galaxy clusters that might generate tidal effects or enrich the intergalactic medium. From these I have measured the spectra of 35 objects, using theWiFeS IFU spectrograph on the ANU 2.3m telescope at Siding Spring. In analysing spectra extracted from the WiFeS data cubes, I found that standard ‘strong line’ methods using emission line ratios to measure atomic abundances, gave either erratic or no results. I found that for those galaxies showing the [O iii] 4363Å auroral line, the metallicities determined using the standard ‘electron temperature’ methodwere inconsistent with previous published work. This led me to investigate the conventional assumption that electrons in Hii regions are in thermal equilibrium. I show that the non-equilibrium ‘ ’ electron energy distribution, found almost universally in solar system plasmas, can explain the long recognised ‘abundance discrepancy’ between recombination line and collisional line abundance calculations in nebular metallicity measurements. This has added an important new dimension to the analysis of nebular spectra. Using the extensively revised Mappings photoionisation modelling code and new atomic data to analyse the spectra of two exceptionally isolated dwarf galaxies, I find that they exhibit metallicities similar to galaxies in more crowded environments, and appear to have evolved quite normally, through periodic star formation and subsequent enrichment of their interstellar media. I present a new approach for calculating total oxygen abundance using electron temperatures that appears to give more consistent results than earlier methods. I apply this to my measured spectra, together with the revised Mappings photoionisation modelling code, to explore the physical parameters affecting the measurement of nebular metallicities. In particular, I find strong evidence for several of the observed nebulae being—in part—optically thin. I use the models to show that nebular optical depth affects measured abundances and temperatures, and that electron densities also have an important role. I develop models that give a very good match to the observations. I conclude that the measurement of abundances and temperatures in Hii regions is a more complex question than had generally been assumed, and important physical parameters affecting the measurement processes have in the past not been taken fully into account.