The Old Halo metallicity gradient: the trace of a self-enrichment process

Based on a model of globular cluster self-enrichment published in a previou s paper, we present an explanation for the metallicity gradient observed th roughout the galactic Old Halo. Our self-enrichment model is based on the a bility of globular cluster progenitor clouds to retain the ejecta of a firs t generation of Type II Supernovae. The key point is that this ability depe nds on the pressure exerted on the progenitor cloud by the surrounding prot ogalactic medium and therefore on the location of the cloud in the protoGal axy. Since there is no significant (if any) metallicity gradient in the who le halo, we also present a review in favour of a galactic halo partly built via accretions and mergers of satellite systems. Some of them bear their o wn globular clusters and therefore "contaminate" the system of globular clu sters formed "in situ", namely within the original potential well of the Ga laxy. Therefore, the comparison between our self-enrichment model and the o bservational data should be limited to the genuine galactic globular cluste rs, the so-called Old Halo group.