Slowly cooking galaxies

Recent spectroscopic observations of IZw 18 have revealed homogeneous abund ance throughout the galaxy and several observations of other starburst gala xies have shown no significant gradient or discontinuity in the abundance d istributions within the HII regions. I thus concur with Tenorio-Tage G., 19 96, AJ 111, 1641 and Devost D., Roy J.R., Drissen L., 1997, ApJ 482. 765, t hat these observed abundance homogeneities cannot be produced by the materi al ejected from the stars formed in the current burst and result from a pre vious star-formation episode. Metals ejected in the current burst of star f ormation remain most probably hidden in a hot phase and are undetectable us ing optical spectroscopy. Combining various observational facts, for instan ce, the faint star-formation rate observed in low surface brightness galaxi es,Van Zee L., Haynes M.P., Salzer J.J., Brocils A.H., 1997c, AJ 113, 1618. 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 evolut ion 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. (C) 2000 Published by Elsevier Science B.V.