Spatially resolved optical and near-infrared spectroscopy of I Zw 18

We present long-slit optical and near-infrared spectroscopy of I Zw 18, one of the most oxygen poor H II galaxies known. We derive physical conditions and oxygen, nitrogen, sulfur, and helium abundances for both the NW (bright) and SE (faint) components from their emission-line spectra. While the value of the oxygen abundance for the NW component derived here is similar to that derived in previous studies, we find a significantly lower nitrogen abundance for the NW component of log (N/O) = -1.56 ± 0.09. This is closer to the typical value found in dwarf irregular galaxies. The discrepancy is attributed, at least in part, to underlying, broad Hα emission, which may be due to embedded supernova remnants. Our sulfur abundance measurement gives log (S/O) = -1.69 ± 0.05, close to the solar value and in agreement with Garnett. We derive a helium abundance of He/H = 0.075 ± 0.002, which represents a value of the helum mass fraction of Y = 0.231 ± 0.006. Our value of Y is slightly lower than some previously determined values, in part, because we do not add a correction for unobserved neutral helium, but it is in excellent agreement with the recent determination by Pagel et al. We draw attention to the fact that using the new He emissivities of Smits will raise the derived helium abundances for low-metallicity dwarfs by as much as 4%. For example, the new He emissivities yield a helium mass fraction of 0.238 for I Zw 18 from the present data. All of the elemental abundances for the two components of I Zw 18 are, within errors, equal, placing a constraint on theories of the evolution of the I Zw 18 system.