Abundances in the Neutral Interstellar Medium of I Zw 18 from Far Ultraviolet Spectroscopic Explorer Observations

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 ratio. Together with a larger spectral coverage (λλ = 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 I absorption is interstellar with a column density of 2.2 × 10²¹ cm^-2. A conservative 3 σ upper limit of 5.25 × 10¹⁴ cm^-2 is derived 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] = -2.06 ± 0.28, [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 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 1 Gyr that enriched the H I phase. Around 470 SNe Ia are required to produce the iron content. A more recent episode that started 10 to several hundred megayears ago is responsible for the additional enrichment of α-elements and nitrogen in the H II regions.