Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705

• Published in: The Astrophysical journal 2004-10, Vol.614 (2), p.698-715
• Main Authors: Lee, Henry, Skillman, Evan D
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/423735

We report optical spectroscopy of 16 H II regions in NGC 1705 and [O III] lambda 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 plus or minus 0.05, which corresponds to [O/H] = -0.45, or 35% of the solar value. There are no significant spatial inhomogeneities in [O III] lambda 4363 oxygen abundances from H II regions at a radius approximately 10" from the super star cluster. In H II regions where [O III] lambda 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 lambda 4686 emission is found in two H II regions, but the implied contribution from O super(+3) to the total oxygen abundance is only 0.01 dex. The mean argon-, neon-, and nitrogen-to-oxygen abundance ratios are consistent with mean values for other dwarf irregular galaxies, blue compact dwarf galaxies, and H II galaxies at comparable oxygen abundances. 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 6 lower than that of the ionized gas. This may be indicative of low-metallicity gas in the halo of the galaxy. Extinction values, A sub(V), derived from observed Balmer line ratios along lines of sight to H II regions are in the range between 0 and 0.9 mag. Significant and variable extinction may have important effects on the interpretation of resolved stellar populations and derived star formation histories. With respect to the metallicity-luminosity and metallicity-gas fraction diagnostics, the measured oxygen abundance for NGC 1705 is comparable to those of Local Group dwarf irregular galaxies at a given luminosity and gas fraction. Simple chemical evolution models suggest that the galaxy is quickly evolving into a gas-poor dwarf galaxy.