Pop III stars and the earliest phases of the evolution of galaxies and IGM

We discuss the effects of very massive Population III stars on the chemical evolution of the Milky Way, elliptical galaxies and the intergalactic medium (IGM) at high reshift. It is shown that the effects produced by Pop III stars on the early evolution of the most common chemical abundances (C, N, O, α-elements, Fe) are negligible if these stars formed only for a very short period of time, corresponding to the suggested threshold metallicity (Z_thr ∼ 10^-4Z_⊙). For a higher threshold metallicity and therefore a longer period of time, the predicted results are at variance with observations. It is also concluded that the IGM at high redshift (z=5.0) cannot have been enriched only by very massive Pop III stars, but that the contribution of lower mass stars is necessary. The same conclusion holds for DLA systems at high redshift.