Metallicity Gradients in Globular Cluster Systems : the Trace of a Self-Enrichment Process ?

We have developed a model of globular cluster self-enrichment, based on the ability of the globular cluster gaseous progenitors to retain the ejecta of a first generation of Type II Supernovae. The key point is that this ability depends on the pressure exerted on the progenitor cloud by the surrounding protogalactic medium and therefore on the location of the cloud in the protogalaxy. The model is able to explain the galactic halo metallicities and the metallicity gradient of the Old Halo which is thought to be the genuine galactic globular cluster system. The possibility that metallicity gradients are a common property of extragalactic globular cluster systems is also presented.