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M. Loewenstein (NASA/GSFC and Univ. of Maryland)
Intracluster medium abundances are higher than expected assuming enrichment by supernovae with progenitors belonging to the simple stellar population observed in cluster galaxies, if stars formed with a standard initial mass function. Moreover, new results on ICM oxygen abundances imply that nucleosynthesis occurred with nonstandard yields. The hypothesis that hypernovae in general, and those associated with Population III stars in particular, may significantly contribute to ICM enrichment is presented and evaluated. The observed abundance anomalies can be explained by a hypernovae-producing subpopulation, but only if it accounts for half of all supernova explosions and if Type Ia supernova rates are very low. Also, the implied energy release may be excessive. However, an independent Pop III contribution -- in the form of metal-free, very massive stars that evolve into hypernovae -- can also account for all the observed abundances, while avoiding these drawbacks and accommodating a normal IMF in subsequent stellar generations. The required number of Pop III stars provides sufficient energy injection (at high redshift) to explain the ICM ``entropy floor''. Pop III hypernova pre-enrich the intergalactic medium, and can produce a significant fraction of the metals observed in the Lyman-alpha forest. Several testable predictions for ICM and IGM observations are made.