FUSE observations of the H I interstellar gas of I Zw 18

Abstract

We present the analysis of FUSE observations of the metal-deficient dwarf galaxy I Zw 18 . We measured column densities of H I, N I, O I, Ar I, Si II, and Fe II. The O I/H I ratio (\log(O I/H I)=-4.7^+0.8_-0.6) is consistent with the O/H ratio observed in the H II regions (all uncertainties are 2-σ). If the oxygen is depleted in the H I region compared to the H II regions, the depletion is at most 0.5 dex. This is also consistent with the \log(O/H) ratios ∼-5 measured with FUSE in the H I regions of other blue compact dwarf galaxies. With \log(N I/O I)=-2.4^+0.6_-0.8, the measured N I/O I ratio is lower than expected for primary nitrogen. The determination of the N II column density is needed to discriminate between a large ionization of N I or a possible nitrogen deficiency. The neutral argon is also apparently underabundant, indicating that ionization into Ar II is likely important. The column densities of the other α-chain elements Si II and Ar I favor the lower edge of the permitted range of O I column density, \log (N_cm^-2(O I))∼ 16.3.