FUSE observations of the HI interstellar gas of I Zw 18

We present the analysis of FUSE observations of the metal-deficient dwarf galaxy I Zw 18. We measured column densities of HI, NI, OI, ArI, SiII, and FeII. The OI/HI ratio (log(OI/HI) = -4.7(-0.7)(+0.8)) is consistent with the O/H ratio observed in the HII regions (all uncertainties are 2-sigma). If the oxygen is depleted in the HI region compared to the HII regions, the depletion is at most 0.5 dex. This is also consistent with the log(O/H) ratios similar to-5 measured with FUSE in the HI regions of other blue compact dwarf galaxies. With log(NI/OI) = -2.4(-0.8)(+0.6), the measured NI/OI ratio is lower than expected for primary nitrogen. The determination of the NII column density is needed to discriminate between a large ionization of NI or a possible nitrogen deficiency. The neutral argon is also apparently underabundant, indicating that ionization into ArII is likely important. The column densities of the other alpha-chain elements SiII and ArI favor the lower edge of the permitted range of OI column density, log(Ncm-2(OI)) similar to 16.3.