FUSE observations of the interstellar gas of I Zw 18

¹ Institut d'Astrophysique de Paris, CNRS, 98 bis Bld Arago, 75014 Paris, France
² LERMA, Observatoire de Paris – Section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France

Corresponding author: A. Lecavelier des Etangs, lecaveli@iap.fr

Received: 28 May 2003
Accepted: 20 September 2003

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 ((O  i/H i) 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 (O/H) ratios ~-5 measured with FUSE in the H i regions of other blue compact dwarf galaxies. With (N i/O  i, 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, (O  i.