Continuous star formation in IZw18

Abstract

We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption of a continuous star formation during bursts. We adopt a 2-D hydrocode coupled with detailed chemical yields originating from SNeII, SNeIa and from single intermediate-mass stars. Different nucleosynthetic yields and different IMF slopes are tested. In most of the explored cases, a galactic wind develops, mostly carrying out of the galaxy the metal-enriched gas produced by the burst itself. The chemical species with the largest escape probabilities are Fe and N. Consequently, we predict that the [α/Fe] and [α/N] ratios outside the galaxy are lower than inside. In order to reproduce the chemical composition of IZw18, the best choice seems to be the adoption of the yields of Meynet & Maeder (2002) which take into account stellar rotation, although these authors do not follow the whole evolution of all the stars. Models with a flat IMF (x = 0.5) seem to be able to better reproduce the chemical properties of IZw18 but they inject in the gas a much larger amount of energy and the resulting galactic wind is very strong, at variance with observations. We also predict the evolution of the abundances in the Hi medium and compare them with recent FUSE observations.