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A. Aloisi, S. Savaglio, T. M. Heckman, C. G. Hoopes (Johns Hopkins University), C. Leitherer, K. R. Sembach (Space Telescope Science Institute)
We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy I~Zw~18. The new data represent an improvement over previous FUSE spectra by a factor of 1.7 in the signal-to-noise. Together with a larger spectral coverage (917-1188 Å), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy.
The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H~{\sc i} absorption is interstellar with a column density of (2.2 ± 0.6) \times 1021 cm-2. A new more stringent constraint is derived for the molecular content, and a conservative 3\sigma upper limit of 5.25 \times 1014 cm-2 is obtained for the column density of diffuse H2. From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = -1.76 ± 0.12, [O/H] = -2.24 ± 0.17, [Si/H] = -2.09 ± 0.12, [Ar/H] = -2.27 ± 0.13, and [N/H] = -2.88 ± 0.11. This is in general several times lower than in the H~{\sc ii} regions. The only exception is iron, whose abundance is the same.
The abundance pattern of the interstellar medium suggests ancient star-formation activity with an age of at least a Gyr that enriched the H~{\sc i} phase. A more recent episode that started 10 to several 100 Myr ago is responsible for the additional enrichment of \alpha elements and nitrogen in the H~{\sc ii} regions.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.