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

Abstract

We show that the difference between the Ar and Si relative abundance ratio derived from FUSE absorption spectra and from the HII 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 HII regions. It follows that no significant ionization correction as suggested by Izotov and collaborators to describe metal contents in damped Ly-alpha 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 ArI1048 and SiII1020 lines observed by Vidal-Madjar et al., we found that the profiles may be reproduced with log (Ar/Si) ~= - 0.8 and N(SiII) ~= 4 10^{15} cm^{-2}.

arXiv comment4 pages, 2 eps figure, submitted to AA

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• Abundances in the Neutral Interstellar Medium of I Zw 18 from FUSE Observations
• Metal abundances and ionization conditions in a possibly dust-free damped Ly-alpha system at z=2.3
• Multiple starbursts in Blue Compact Galaxies
• Continuous star formation in IZw18
• FUSE observations of the HI interstellar gas of IZw18
• The First Detection of Cobalt in a Damped Lyman Alpha System
• Ar I as a tracer of ionization evolution
• Cosmological Evolution of Heavy Element and Molecular Hydrogen Abundances
• Metal abundances and kinematics of quasar absorbers - II. Absorption systems toward Q0347-3819 and APM BR J0307-4945