Infrared Space Observatory Short Wavelength Spectrometer Observations of H II Regions in NGC 6822 and I Zwicky 36: Sulfur Abundances and Temperature Fluctuations

Abstract

We report Infrared Space Observatory (ISO) Short Wavelength Spectrometer infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Brα, [S III] at 18.7 and 33.5 μm, and [S IV] at 10.5 μm are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S⁺³ abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S⁺³ abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] λ9532 or λ9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.