A Survey for Supernovae in Massive High-Redshift Clusters
  We describe our ongoing program designed to measure the SN-Ia rate in a sample of massive z=0.5-0.9 galaxy clusters. The SN-Ia rate is a poorly known observable, especially at high z, and in cluster environments. The SN rate and its redshift dependence can serve as powerful discrimiminants for a number of key issues in astrophysics and cosmology. Our observations will put clear constraints on the characteristic SN-Ia ``delay time'', the typical time between the formation of a stellar population and the explosion of some of its members as SNe-Ia. Such constraints can exclude entire categories of SN-Ia progenitor models, since different models predict different delays. These data will also help to resolve the question of the dominant source of the high metallicity in the intracluster medium (ICM) - SNe-Ia, or core-collapse SNe from an early stellar population with a top-heavy IMF, perhaps those population III stars responsible for the early re-ionization of the Universe. Since clusters are excellent laboratories for studying enrichment (they generally have a simple star-formation history, and matter cannot leave their deep potentials), the results will be relevant for understanding metal enrichment in general, and the possible role of first generation stars in early Universal enrichment. Observations obtained so far during cycles 14 and 15 yield many SNe in our cluster fields, but our follow-up campaign reveals most are not in cluster galaxies. 
Keren Sharon (TAU), A. Gal-Yam (Caltech), D. Maoz (TAU), M. Donahue   (MSU), H. Ebeling (IfA), R. Ellis (Caltech), A. Filipp...