Abstract
ABSTRA C T 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 starformation history of the Universe is about 50 per cent at za 0; 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 kZla V ej =Vb . 1=25Z( at za 3: 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 z & 1; as assumed, at za 0 the metallicity of the IGM is narrowly confined within the range Z < 0:1 ^ 0:03Z(: Finally, we point out that our results can account for the observed metal content of the intracluster medium.
Highlights
Primordial nucleosynthesis enriched gas in the universe with the light elements He, D, and Li
The basic conclusion of this paper is that metal ejection driven by SN events fails, by more than one order of magnitude, to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole intergalactic medium (IGM) to the typical metallicity of Lyα clouds, [C/H] ≃ −2.5
We postulate that a second process is responsible for the diffusion of metals on a typical scale Rd, comparable to the mean distance between the galaxies which are the most efficient pollutants of the IGM
Summary
Primordial nucleosynthesis enriched gas in the universe with the light elements He, D, and Li. The common assumption that supernova driven winds may be able to distribute metals over large distances has been shown to be too simplistic and does not stand up to close quantitative scrutiny (MacLow & Ferrara 1999; Murakami & Babul 1999; D’Ercole & Brighenti 1999) These studies have come to the conclusion that efficient blowout is likely to be inhibited by the galaxy ISM and, at least at high redshift where densities are higher, by the pressure of the surrounding intergalactic gas (Babul & Rees 1992; Ciardi & Ferrara 1997).
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