Argon and Silicon abundances in the damped Ly α system I Zw 18

We show that the difference between the Ar and Si relative abundance ratio derived from FUSE absorption spectra and from the H ii regions of I Zw 18 is a consequence of the microturbulent analysis applied to the absorption spectra. FUSE observations were performed with a large entrance aperture which fully covered the galaxy. This means that the observed profiles are averaged over the full body of I Zw 18, implying that large-scale velocity fields influence the absorption-line profiles. Taking this into account, we show that the absorption spectra are consistent with the same metal abundances as those derived from the H ii regions. It follows that no significant ionization correction as suggested by Izotov and collaborators to describe metal contents in damped Lyα systems (DLA) is required to model abundances in the neutral gas of I Zw 18 (a local DLA system). Using a mesoturbulent approach and applying the generalized radiative transfer equation to the Ar iλ1048 and Si iiλ1020 lines observed by Vidal-Madjar et al., we found that the profiles may be reproduced with log (Ar/Si) ≃ −0.8 and N(Si ii) ≃ 4×1015 cm.