Mixing metals in the early Universe

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 SNae 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, a additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies which 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% at z = 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 Z = Ω ej Z /Ω b ≃ 1/25Z ⊙ at z = 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 z = 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.

Cosmic metal enrichment

“…First, metals produced by LBGs do not seem to be able to escape from their halos, due to the confining mechanisms mentioned above. This is consistent with the prediction (Ferrara, Pettini & Shchekinov 2000) that galaxies of total mass M > 10 12 (1 + z) −3/2 M do not eject their metals into the IGM. Interestingly, the metallicity-mass relation recently derived from the SDSS (Tremonti et al 2004) shows that galaxies with stellar masses above 3 × 10 10 M (their total mass corresponds to M for a star formation efficiency f = 0.2) chemically evolve as "closed boxes," i.e.…”

“…the metal escape fraction, δ B . A solution has been proposed by Ferrara, Pettini & Shchekinov (2000) who noticed that SNe in "normal" galaxies (as for example the Milky Way) are distributed in the disk and clustered in OB associations; thus, these explosion sites act incoherently. In low-mass galaxies, on the contrary, the size of the galaxy is comparable to the size of individual SN-driven bubbles and therefore the energy deposited can work to drive coherently the same outflow.…”

KODIAQ-Z: Metals and Baryons in the Cool Intergalactic and Circumgalactic Gas at 2.2 ≲ z ≲ 3.6

“…It is also much smaller for the LYAF (e.g., Simcoe et al 2004), implying overall a more chemically homogeneous gas in the LYAF and DLA regimes. Therefore, in and quite near galaxies, in the gas traced by DLAs, the metal enrichment from supernovae is quite efficient, but beyond the immediate vicinity of the galaxies, the volume filling factor for metals must be low, which is consistent with models of metal ejection from supernovae that is not expected to be efficient and confined to small regions (e.g., Ferrara et al 2000;Scannapieco 2005).…”

“…The larger fraction of metal-enriched and even supermetallicity absorbers at low redshift indicates that a much larger volume of the universe has been exposed to metal pollution than at 2.2  z  3.6. Ferrara et al (2000) developed a model that predicts that at z < 1 essentially all absorbers should have associated metal absorption, and the spread in metallicity should be less than 1 dex. This is not the case.…”

Pair-production supernovae: Theory and observation

“…In particular the only difference between PopIII and PopII/I outflows arises from the energy input per unit gas mass," E III,II g . In the PopII/I case, we calculated this assuming that 10% of the gas was converted into stars, that 10 51 ergs of kinetic energy input 300 M ⊙ of stars formed, and we assumed an overall wind efficiency fit to results of Mori, Ferrara, & Madau (2002) and Ferrara, Pettini, & Shchekinov (2000). In the PopIII case, on the other hand, as there are no direct constraints, we varied E III g over a large range as in §5.…”