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 opt ical imaging has revealed an H II region in UGC 4483 with an Halpha fl ux of almost-equal-to 1 X 10(-13) ergs cm-2 s-1. Optical and near-infr ared spectroscopy of this H II region yields He, N, O, Ne, and S abund ances for the interstellar matter (ISM) in this galaxy. The spectra we re acquired with several different telescope/instrument combinations i n order to assess the quality of the derived uncertainties. With an ox ygen abundance of 3.3 x 10(-5) [12 + log (O/H) = 7.5], this galaxy is similar to GR 8 and among the most metal poor dwarf irregulars known t o date. However, the H II region in UGC 4483 has high excitation and h igher surface brightness than GR 8, allowing very accurate abundance e stimates. The N/O ratio is 3%, in good agreement with other low-metall icity 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-met allicity dwarf irregulars. A He/H abundance of 0.079 +/- 0.002 is deri ved from observations of the lambda6678 He I emission line. A comparis on of observations obtained with different telescopes and with the sam e telescope on different nights supports our relatively small estimate d uncertainty in this measurement. We determine that the neutral heliu m fraction is insignificant through both direct observations of the He +/H+ ratio across the nebula and through photoionization modeling. Thi s results in a helium mass fraction of 0.239 +/- 0.006, consistent wit h the most recent theoretical and observational determinations of the primordial helium abundance. We also discuss remaining systematic unce rtainties in the calculation of the primordial helium abundance.