The Main Epoch of Metal Production in the Universe

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 (∼ 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 > ∼ 3). Since such stars account for at least ∼ 30% of the baryons now locked into stars, it is argued that at least 30% of stars and metals formed before z ≃ 3, and correspondingly the metallicity of the universe at z = 3 is predicted to be ∼ 1/10 solar.