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D.J. Bomans (Astronomisches Institut der Ruhr-Universitaet Bochum)
The dwarf galaxy I Zw 18 is since its discovery in 1972 and despite subsequent large survey efforts still the most metal poor galaxy known today. Interestingly, the oxygen abundance between different positions in the galaxy is effectively the same, excluding self-enrichment from the current population of young stars as source of the metals. The question if I Zw 18 is an truly young galaxy or evolved very slowly is still open, even with several recent observations favor the slow-evolution scenario. I Zw 18 currently forms stars at high rate, which apparently leads to large gaseous shells and outflows extending several kpc away from the galaxy as seen in H\alpha images. At least three of the gas shells expand away from I Zw 18, removing this gas from the sites of possible future star formation, consistent with a slow-evolution scenario. For two shells we could make an association with diffuse X-ray emission using the ROSAT HRI. If this hot gas can be shown to be metal loaded, as expected from super-bubble theory, its properties would present an observational key to test the chemical evolution of dwarf galaxies. Such hot metal-enriched galactic winds were postulated by chemical evolution models for quite some time, but not proven by direct observations in low metalicity dwarf galaxies yet. In this paper will present the analysis and results of archival HST imaging and spectroscopy, our ROSAT HRI data, and our new CHANDRA images/spectra and will draw conclusions on the evolutionary history and current state of I Zw 18.
This research was supported by Verbundforschung grant DLR 50 OR 9906 4.