Recent spectroscopic observations of IZw~18 have revealed homogeneous
abundance throughout the galaxy and several observations of other starburst
galaxies have shown no significant gradient or discontinuity in the abundance
distributions within the HII regions. I thus concur with Tenorio-Tagle (1996)
and Devost et al. (1997) that these observed abundance homogeneities cannot be
produced by the material ejected from the stars formed in the current burst and
result from 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. Combining various observational facts,
for instance the faint star formation rate observed in low surface brightness
galaxies (van Zee et al., 1997), I propose that a low and continuous star
formation rate occurring during quiescent phases between bursts is a non
negligible source of new elements in the interstellar medium. Using a
spectrophotometric and chemical evolution model for galaxies, I investigated
the star formation history IZw~18. I demonstrate that the continuous star
formation scenario reproduces all the observed parameters of IZw~18. I discuss
the consequences of such a quiet star formation regime.Comment: Proceedings of the JENAM Conference (Toulouse, September 1999). To be
published in New Astronomy Reviews, Editors Daniel Schaerer and Rosa Gonzalez
Delgado. 8 pages, 3 figure
