A continuous low star formation rate in IZw 18?

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 a bundance 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 obse rved metals were formed in a previous star formation episode. Metals ejecte d in the current burst of star formation remain most probably hidden in a h ot phase and are undetectable using optical spectroscopy. We discuss differ ent scenarios of star formation in IZw 18. Combining various observational facts, for instance the faint star formation rate observed in low surface b rightness galaxies (Van Zee et al. 1997c), it is proposed that a low and co ntinuous rate of star formation occurring during quiescent phases between b ursts could be a significant source of metal enrichment of the interstellar medium.