Interstellar Medium Abundances in Sculptor Group Dwarf Irregular Galaxies

Using the Cerro Tololo Inter-American Observatory 4 m telescope, we have obtained optical spectra of H II regions in five Sculptor group dwarf irregular galaxies. We derive oxygen, nitrogen, and sulfur abundances from the H II region spectra. Oxygen abundances are derived via three different methods (the "direct" method, the empirical method guided by photoionization modeling of McGaugh [published in 1991], and the purely empirical method of Pilyugin, published in 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 interstellar medium 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 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. Measurements of the H II regions in NGC 625 yield log (N/O) ≈ -1.25. Assuming N production by intermediate-mass stars, this relatively high N/O ratio may be indicative of a long quiescent period prior to the recent active burst of star formation. The oxygen abundances in the Sculptor group dI's are in good agreement with the relationship between metallicity and luminosity observed in the Local Group dI's. Taken together, the observations show a better relationship between metallicity and luminosity than between metallicity and galaxy central surface brightness. The Sculptor group dI's, in general, lie closer to the simple closed-box model evolutionary path than the Local Group dI's. The higher gas contents, lower average star formation rates, and closer resemblance to closed-box evolution could all be indicative of evolution in a relatively low density environment.