The Cosmic Infrared Background Radiation, Star Formation Rate, and   Metallicity
  The density of the far infrared / submillimeter (FIR/SMM) diffuse extragalactic radiation field has recently been determined from COBE data. Nearly simultaneously, deep FIR/SMM surveys have detected substantial numbers of optically unidentified sources, which have led to the proposal that galaxies and protogalaxies at red shifts z = 2 to 4 may account for an appreciable fraction of the background. Here, I show that, if the reported radiation levels are generated through nucleosynthesis, most of this energy must have been produced at epochs $z \lesssim 2$. Hubble Deep Field data cited by Madau et al. (1998) indicate that the bulk of the integrated extragalactic background must have been generated even more recently at $z < 1$. 
Martin Harwit