Hints on Galaxy Formation from the Metal Content of Galaxy Cluster and Other Fossil Evidence
Clusters of galaxies allow a direct estimate of the metallicity and metal production yield on the largest scale so far. It is 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 not only in cluster ellipticals but also 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 $z\simeq 3$, and correspondingly 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.