ON THE ENRICHMENT OF THE INTERGALACTIC MEDIUM BY GALACTIC WINDS

Observations of metal lines in Ly alpha absorption systems of small H I column density and their ubiquitous nature suggest that the intergal actic medium (IGM) was enriched to about Z similar to 0.01 Z. by a red shift z similar to 3. We investigate the role of winds from small star -forming galaxies at high z in enriching the IGM. The existence of lar ge numbers of small galaxies at high z follows naturally from hierarch ical clustering theories (e.g., CDM). For analytical simplicity we ass ume that the galactic winds escape the galaxies at a single characteri stic redshift z(in), and we model the galactic winds as spherical shoc k waves propagating through the IGM. We then calculate the probability distribution of the metallicity of the IGM, as a function of time (fo r different values of z(in)), adopting plausible galaxy mass functions (from Press-Schechter formalism), cooling physics, star formation eff iciencies, gas ejection dynamics, and nucleosynthesis yields. We compa re this expected distribution with the observed distribution of metall icities 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 su ch a scenario. We find that galactic winds at high z could have enrich ed the IGM to a mean metallicity of Z similar to 0.01 Z, at z similar to 3, with a standard deviation of the same order, if z(in) less than or similar to 5, and that this satisfies all the observational constra ints.