ISM Abundances in Sculptor Group Dwarf Irregular Galaxies

Using the CTIO 4-m telescope, we have obtained optical spectra of HII regions in five Sculptor Group dwarf irregular galaxies. We derive oxygen, nitrogen, and sulfur abundances from the HII region spectra. Oxygen abundances are derived via three different methods (the “direct” method, the empirical method guided by photoionization modeling of McGaugh (1991), and the purely empirical method of Pilyugin (2000)) and are compared. Significant systematic differences are found between the three methods, and we suggest that a recalibration of the empirical abundance scale is required. Until differences between these three methods are better understood, the issue of the degree of uniformity of the ISM abundances in a dwarf galaxy cannot be properly addressed. The N/O ratio for the metal-poor dI ESO 473-G24 of log (N/O) = −1.43 ± 0.03 lies well above the plateau of log (N/O) = −1.60 ± 0.02 found by Izotov & Thuan (1999) for a collection of metal-poor blue compact galaxies. This shows that not all galaxies with 12 + log (O/H) ≤ 7.6 have identical elemental abundance ratios, and this implies that the Izotov & Thuan scenario for low metallicity galaxies is not universal. Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.