On Iron Enrichment, Star Formation, and Type Ia Supernovae in Galaxy Clusters

The nature of star formation and Type Ia supernovae (SNIa) in galaxies in thefield and in rich galaxy clusters are contrasted by juxtaposing the build-up ofheavy metals in the universe inferred from observed star formation andsupernovae rate histories with data on the evolution of Fe abundances in theintracluster medium (ICM). Models for the chemical evolution of Fe in theseenvironments are constructed, subject to observational constraints, for thispurpose. While models with a mean delay for SNIa of 3 Gyr and standard initialmass function (IMF) are consistent with observations in the field, cluster Feenrichment immediately tracks a rapid, top-heavy phase of star formation --although transport of Fe into the ICM may be more prolonged and star formationlikely continues to redshifts <1. The source of this prompt enrichment is TypeII supernovae (SNII) yielding at least 0.1 solar masses per explosion (if theSNIa rate normalization is scaled down from its value in the field according tothe relative number of candidate progenitor stars in the 3-8 solar mass range)and/or SNIa explosions with short delay times associated with the rapid starformation mode. Star formation is >3 times more efficient in rich clusters thanin the field, mitigating the overcooling problem in numerical clustersimulations. Both the fraction of baryons cycled through stars, and thefraction of the total present-day stellar mass in the form of stellar remnants,are substantially greater in clusters than in the field.Comment: 51 pages including 26 figures and 2 tables, accepted for publication in ApJ 5/4/0