Sources for Metals in the Intergalactic Medium
  We present a discussion of possible sources of C, O, and Si in the IGM using the yields of VMSs and SNe II. The chemical evolution of the IGM is considered based on analytical phenomenological models of hierarchical structure formation. Two regimes are considered: one for gas expulsion by VMSs and SNe II in low-mass halos prior to dissociation of H2 molecules and reionization, and the other for later SN II-driven outflows from intermediate-mass halos. We use recent data on the abundances of C, O, and Si in the IGM inferred from two UVB models. We show that the results from a UVB model including quasars only cannot be explained by existing stellar models. To account for the results, in particular [Si/C], from a softer UVB model requires VMSs to provide more than 15% of the C in the IGM. The preferred scenario is that VMSs in low-mass halos provided between 15% and 60% of the C in the regime of very high redshift (z>15) and that galactic outflows provided the remainder during later epochs (4<z<6). Thus, there is a large gap in z between metal production by VMSs very early in the chemical evolution of the IGM and subsequent contributions from galactic outflows. (Abridged) 
Y.-Z. Qian (UMN), G. J. Wasserburg (Caltech)