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Mixing metals in the early Universe

Authors

  • Andrea Ferrara,

    Corresponding author
    1. 1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA3 Department of Physics, Rostov State University, 194 Stachki, 344104 Rostov on Don, Russia
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  • 1 Max Pettini,

    1. 1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA3 Department of Physics, Rostov State University, 194 Stachki, 344104 Rostov on Don, Russia
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  • and 2 Yuri Shchekinov 1,3

    1. 1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA3 Department of Physics, Rostov State University, 194 Stachki, 344104 Rostov on Don, Russia
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★ E-mail: ferrara@arcetri.astro.it

Abstract

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two-step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings 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 distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, an additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies that are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the star-formation history of the Universe is about 50 per cent at inline image that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the Universe, the average IGM metallicity would be inline image at inline image However, due to spatial inhomogeneities, the mean of the distribution 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 inline image as assumed, at inline image the metallicity of the IGM is narrowly confined within the range inline image Finally, we point out that our results can account for the observed metal content of the intracluster medium.

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