Evidence for Abundance Differences between H i and H ii Regions in Dwarf Galaxies

Abstract. We present FUSE spectroscopic study of the nearby metal–deficient (Z ⊙ /14) blue compact dwarf (BCD) galaxy I Zw 36. Atomic hydrogenand many metal lines are observed in absorption against the stellar continuumof young, massive stars embedded in ionized regions. Profile fitting of absorp-tion lines allowed us to investigate the chemical composition of the neutral gas.This study present strong evidences that nitrogen and argon are underabundantin the neutral gas as compared to the ionized gas. The estimation of oxygenabundance using phosphorus as a tracer also leads to an underabundance in theneutral gas. By comparing these results with those of other BCDs observedwith FUSE, we conclude that underabundances can be accounted for by a fastself–enrichment of the ionized regions. 1. Data analysisI Zw 36 was observed by FUSE (Moos et al. 2000) in the range 905−1187 ˚Awith the 30”×30” aperture (Lebouteiller et al. 2003).Column densities are calculated using the profile fitting procedure Owensde-veloped at the Institut d’Astrophysique de Paris by Martin Lemoine and theFUSE French team (see e.g. H´ebrard et al. 2002). This program returns themost likely values of free parameters such as Doppler widths (b), velocities (v),and column densities through a χ