Chemical Abundances of HiiRegions in the Starburst Galaxy NGC 1705

Abstract

(Shortened version) We report optical spectroscopy of 16 H II regions in NGC 1705 and [O III] 4363 detections for the first time in five H II regions. The resulting mean oxygen abundance derived directly from measured electron temperatures is 12+log(O/H) = 8.21 +/- 0.05, which corresponds to [O/H] = -0.45, or 35% of the solar value. There are no significant spatial inhomogeneities in [O III] 4363 oxygen abundances from H II regions at a radius approximately 10 arcsec from the super star cluster. In H II regions where [O III] 4363 was not measured, oxygen abundances derived with bright-line methods (accurate only to 0.2 dex) are in agreement with direct values of the oxygen abundance. Faint narrow He II 4686 emission is found in two H II regions, but the implied contribution from O^+3 to the total oxygen abundance is only 0.01 dex. Interestingly, the nitrogen-to-oxygen abundance ratio in the ionized H II gas agrees with the value for the neutral H I, even though the metallicity of the neutral gas may be a factor of six lower than that of the ionized gas. This may be indicative of low-metallicity gas in the halo of the galaxy. Extinction values, A_V, derived from observed Balmer line ratios along lines of sight to H II regions are in the range between zero and 0.9 mag. With respect to the metallicity-luminosity and metallicity-gas fraction diagnostics, the measured oxygen abundance for NGC 1705 is comparable to Local Group dwarf irregulars at a given luminosity and gas fraction.