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Issue A&A
Volume 413, Number 1, January I 2004
Page(s) 131 - 137
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20031518



A&A 413, 131-137 (2004)
DOI: 10.1051/0004-6361:20031518

FUSE observations of the $\ion{H}{I}$ interstellar gas of I Zw 18

A. Lecavelier des Etangs1, J.-M. Désert1, D. Kunth1, A. Vidal-Madjar1, G. Callejo1, 2, R. Ferlet1, G. Hébrard1 and V. Lebouteiller1

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

(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 ( $\log$(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- $\sigma$). 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 $\alpha$-chain elements Si II and Ar I favor the lower edge of the permitted range of O I column density, $\log (N_{\rm cm^{-2}}$(O I $))\sim 16.3$.


Key words: line: profiles -- galaxies: abundances -- galaxies: dwarf -- galaxies: ISM -- galaxies: individual: I Zw 18 -- ultraviolet: galaxies

Offprint request: A. Lecavelier des Etangs, lecaveli@iap.fr

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