Supernova rates: seeing cosmic metal enrichment in action
The measurement of supernova (SN) rates holds great promise, both for tracing the history of cosmic metal enrichment and for understanding the explosions themselves and their progenitor systems. I will describe several ongoing observational programs to measure the rates of type-Ia and core-collapse SNe, as a function of redshift and environment. Based on measurements from the ground and with HST, the SN-Ia rate in galaxy clusters remains low out to redshift z~1, showing that the large mass of intracluster metals was produced by an early population with a short SN delay time. However, the simplest combinations of SN models and cluster star-formation histories fail to reproduce the observed SN-Ia rate vs. redshift. In the field, I will describe a program with Subaru that aims to measure the SN rate out to z~2, using novel methodologies for SN classification. First results suggest a SN-Ia rate that may be tracking the star-formation history with a small delay, in contrast to the conclusions of the HST-GOODS survey, but in accord with expectations from recent local (z=0) results, and with the cluster results above. Finally, I will discuss a puzzling corollary of all the recent rate measurements: the fraction of the supposed SN-Ia progenitor population that actually explodes as SNe-Ia is suprisingly high -- perhaps higher than possible in any of the currently popular models.