Cosmic Chemical Evolution

Numerical simulations of standard cosmological scenarios have now reached thedegree of sophistication required to provide tentative answers to thefundamental question: Where and when were the heavy elements formed? Averagingglobally, these simulations give a metallicity that increases from 1% of thesolar value at $z=3$ to 20% at present. This conclusion is, in fact,misleading, as it masks the very strong dependency of metallicity on localdensity. At every epoch higher density regions have much higher metallicitythan lower density regions. Moreover, the highest density regions quicklyapproach near solar metallicity and then saturate, while more typical regionsslowly catch up. These results are much more consistent with observational datathan the simpler picture (adopted by many) of gradual, quasi-uniform increaseof metallicity with time.Comment: ApJ(Letters) in press, 15 latex pages and 4 figure