Slowly cooking galaxies

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-Tage G., 1996, AJ 111, 1641 and Devost D., Roy J.R., Drissen L., 1997, ApJ 482. 765, 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 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 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. (C) 2000 Published by Elsevier Science B.V.