Chemical Evolution on the Scale of Clusters of Galaxies, and Beyond

Alvio Renzini

Published 1999-02-25, updated 1999-03-09Version 2

Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. The ratio of the total iron mass in the ICM to the total optical luminosity of the cluster (the iron mass-to-light-ratio) is the same for all clusters which ICM is hotter than $\sim 2$ keV, and the elemental proportions (i.e. the [$\alpha$/Fe] ratio) appear to be solar. From these evidences it is argued that both the IMF as well the relative contributions of SN types are likely to be universal. Constraints on the past SN activity in galaxy clusters are then derived, and support is given to the notion that the average SNIa rate was much higher in the past, i.e. at least 10 times more than currently observed in local ellopticals. It is also argued that cluster metallicity ($\sim 1/3$ solar) should be taken as representative of the low-z universe as a whole. There is now compelling evidence that the bulk of stars in cluster as well as in field ellipticals and bulges formed at high redshifts ($z\gsim 3$). Since such stars account for at least $\sim 30%$ of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before $z\simeq 3$. As a consequence, the metallicity of the universe at z=3 is predicted to be $\sim 1/10$ solar. This requires the cosmic star formation rate to run at least flat from $z\sim 1$ to $\sim 5$, which appears to agree with the most recent derect determinations of the star formation rate in Lyman-break galaxies.