On the enrichment of the intergalactic medium by galactic winds

Abstract

Observations of metal lines in Ly-alpha absorption systems of small H I column density and their ubiquitous nature suggest that the intergalactic medium (IGM) was enriched to about Z about 0.01 Z(solar) by a redshift z about 3. We investigate the role of winds from small star-forming galaxies at high z in enriching the IGM. The existence of large numbers of small galaxies at high z follows naturally from hierarchical clustering theories. For analytical simplicity we assume that the galactic winds escape the galaxies at a single characteristic redshift z(in), and we model the galactic winds as spherical shock waves propagating through the IGM. We then calculate the probability distribution of the metallicity of the IGM, as a function of time, adopting plausible galaxy mass functions, cooling physics, star formation efficiencies, gas ejection dynamics, and nucleosynthesis yields. We compare this expected distribution with the observed distribution of metallicities in the Ly-alpha forest at z = 3, with the metal-poor stars in the halo of our Galaxy, and with other observational constraints on such a scenario. We find that galactic winds at high z could have enriched the IGM to a mean metallicity of Z about 0.01 Z(solar) at z about 3, with a standard deviation of the same order, if z(in) is less than about 5, and that this satisfies all the observational constraints.