Nebular abundances of nearby southern dwarf galaxies

Abstract

The results of optical spectroscopy of H II regions in a sample of southern dwarf irregulars consisting of five dwarf galaxies in the Centaurus A group, four dwarfs in the Sculptor group, and eight additional dwarf galaxies are presented. Oxygen abundances are derived using the direct method where [O III]λ4363 is detected; otherwise, abundances are derived with the bright-line method using the McGaugh and the Pilyugin calibrations. ESO358-G060 has the lowest oxygen abundance (12-log(O/H) = 7.32) in the sample, which is comparable to the value for the second most metal-poor galaxy known (SBS 0335-052). In all, new oxygen abundances are reported for nine dwarf galaxies; updated values are presented for the remaining galaxies. Derived oxygen abundances are in the range from 3% to 26% of the solar value. Oxygen abundances for dwarfs in the southern sample are consistent with the metallicity-luminosity relationship defined by a control sample of dwarf irregulars with [O III]λ4363 abundances and well-measured distances. However, NGC 5264 appears to have an (upper branch) oxygen abundance approximately two to three times higher than other dwarfs at similar luminosities. Nitrogen-to-oxygen and neon-to-oxygen abundance ratios are also reported; in particular, IC 1613 and IC 5152 show elevated nitrogen-to-oxygen ratios for their oxygen abundances.