THE TYPE Ia SUPERNOVA RATE IN REDSHIFT 0.5-0.9 GALAXY CLUSTERS

Supernova (SN) rates are potentially powerful diagnostics of metal enrichment\nand SN physics, particularly in galaxy clusters with their deep,\nmetal-retaining potentials and relatively simple star-formation histories. We\nhave carried out a survey for supernovae (SNe) in galaxy clusters, at a\nredshift range 0.5<z<0.9, using the Advanced Camera for Surveys (ACS) on the\nHubble Space Telescope. We reimaged a sample of 15 clusters that were\npreviously imaged by ACS, thus obtaining two to three epochs per cluster, in\nwhich we discovered five likely cluster SNe, six possible cluster SNe Ia, two\nhostless SN candidates, and several background and foreground events. Keck\nspectra of the host galaxies were obtained to establish cluster membership. We\nconducted detailed efficiency simulations, and measured the stellar\nluminosities of the clusters using Subaru images. We derive a cluster SN rate\nof 0.35 SNuB +0.17/-0.12 (statistical) \\pm0.13 (classification) \\pm0.01\n(systematic) [where SNuB = SNe (100 yr 10^10 L_B_sun)^-1] and 0.112 SNuM\n+0.055/-0.039 (statistical) \\pm0.042 (classification) \\pm0.005 (systematic)\n[where SNuM = SNe (100 yr 10^10 M_sun)^-1]. As in previous measurements of\ncluster SN rates, the uncertainties are dominated by small-number statistics.\nThe SN rate in this redshift bin is consistent with the SN rate in clusters at\nlower redshifts (to within the uncertainties), and shows that there is, at\nmost, only a slight increase of cluster SN rate with increasing redshift. The\nlow and fairly constant SN Ia rate out to z~1 implies that the bulk of the iron\nmass in clusters was already in place by z~1. The recently observed doubling of\niron abundances in the intracluster medium between z=1 and 0, if real, is\nlikely the result of redistribution of existing iron, rather than new\nproduction of iron.\n