Spatially resolved optical and near-infrared spectroscopy of the low-metallicity galaxy UGC 4483

UGC 4483 is a dwarf irregular galaxy in the M81 group. Narrow-band imaging has revealed an H II region in UGC 4483 with an H alpha flux approximately = 1 x 10 (exp - 13) ergs/sq cm/s. Optical and near-infrared spectroscopy of this H ll region yields He, N, O, Ne, and S abundances for the interstellar matter (ISM) in this galaxy. The spectra were acquired with several different telescope/instrument combinations in order to assess the quality of the derived uncertainties. With oxygen abundance of 3.3 x 10 (exp -5) (12 + log (O/H) = 7.5), this galaxy is similar to GR 8 and among the most metal poor dwarf irregulars known to date. However, the H II region in UGC 4483 has high excitation and higher surface brightness than GR 8, allowing very accurate abundance estimates. The N/O ratio is 3%, in good agreement with other low-metallicity dwarf galaxies. The S/O abundance ratio in UGC 4483 is close to the solar ratio, consistent with results for I Zw 18 and other low-metallicity dwarf irregulars. A He/H abundance of 0.079 +/- 0.002 is derived from observations of the lambda 6678 He ll emission line. A comparison of observations obtained with different telescopes and with the same telescope on different nights supports our relatively small estimated uncertainty in this measurement. We determine that the neutral helium fraction is insignificant through both direct observations of the He(+)/H(+) ratio across the nebula and through photoionization modeling. This results in a helium mass fraction of 0.239 +/- 0.006, consistent with the most recent theoretical and observational determinations of the primordial helium abundance. We also discuss remaining systematic uncertainties in the calculation of the primordial helium abundance