Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18 obtained with the CFH 3.6 m Telescope are presented. The very low value of oxygen abundance previously reported is confirmed and a very homogeneous abundance distribution is found (no variation larger than 0.05 dex) over the whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al. (1997) that the observed abundance level cannot result from the material ejected by the stars formed in the current burst, and propose that the observed metals were formed in a previous star formation episode. Metals ejected in the current burst of star formation remain most probably hidden in a hot phase and are undetectable using optical spectroscopy. We discuss different scenarios of star formation in IZw 18. Combining various observational facts, for instance the faint star formation rate observed in low surface brightness galaxies van Zee et al. (1997), it is proposed that a low and continuous rate of star formation occurring during quiescent phases between bursts could be a significant source of metal enrichment of the interstellar medium.
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