Mixing metals in the early Universe

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose tha t metal enrichment occurs as a two-step process. First, supernova (SN) expl osions eject metals into relatively small regions confined to the surroundi ngs of star-forming galaxies. From a comprehensive treatment of blowout we show that SN by themselves fail by more than one order of magnitude to dist ribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, an additio nal (but as yet unknown) physical mechanism must be invoked to mix the meta ls on scales comparable to the mean distance between the galaxies that are most efficient pollutants. From this simple hypothesis we derive a number o f testable predictions for the evolution of the IGM metallicity. Specifical ly, we find that: (i) the fraction of metals ejected over the star-formatio n history of the Universe is about 50 per cent at z = 0; that is, approxima tely half of the metals today are found in the IGM; (ii) if the ejected met als were homogeneously mixed with the baryons in the Universe, the average IGM metallicity would be [Z] = Omega (ej)(Z)/Omega (b) similar or equal to 1/25Z. at z = 3. However, due to spatial inhomogeneities, the mean of the d istribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at z less than or similar to 1, as assumed, at z = 0 the metallicity of the IGM is narrowly confined within the range Z approxi mate to 0.1 +/- 0.032.. Finally, we point out that our results can account for the observed metal content of the intracluster medium.