The metallicity of star-forming gas over cosmic time
  A brief review concludes that there is now good overall agreement between theoretical estimates of the energy associated with the production of the observed metal content of the Universe and the observed extragalactic background light. In addition the overall form of the star-formation history over 0 < z < 5 is reasonably well constrained.   The study of emission line gas in galaxies as a function of redshift provides a complementary view of chemical evolution to that obtained from studies of absorption line systems in quasar spectra. Emission line gas is more relevant for some questions, including the confrontation with models for the chemical evolution of our own and other galaxies, and Origins-related questions about the formation of stars and planets. Using relatively crude diagnostic parameters such as Pagel's R23, the increase of the metallicity of star-forming gas with cosmic epoch can be tracked. Observations of a large sample of CFRS galaxies at z ~ 0.8 (a look-back time of 0.5 t0 show the appearance of a significant number of low metallicity systems amongst luminous L* galaxies. However, overall there is only a modest change in mean metallicity compared with the present, D log Z = -0.08 +- 0.06. These data do not support a fading-dwarf scenario for the ``faint blue galaxies''. At z ~ 3 it is likely that all star-forming galaxies have sub-solar metallicities. The overall increase in the metallicity of star-forming gas with cosmic epoch matches rather well the age-metallicity relation in the solar neighbourhood and some recent models for the evolution of the global metallicity of gas in the Universe. 
Simon J. Lilly, C. Marcella Carollo, Alan N. Stockton