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

We present the analysis of FUSE observations of the metal-deficient dwarf galaxy IZw 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_-0.6^-0.8) is consistent with the O/H ratio observed in the HII 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 Hi regions of other blue compact dwarf galaxies. With log(N I/O I) = -2.4_-0.8^=0.6, 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 a-chain elements Sill and An favor the lower edge of the permitted range of O I column density, log(N _cm-2(O I)) ∼ 16.3.