Metal Enrichment of the Intergalactic Medium at z=3 by Galactic Winds

Studies of quasar absorption lines reveal that the low density intergalactic medium at z ~ 3 is enriched to 0.001-0.01 solar metallicity. This enrichment may have occurred in an early generation of Population III stars at redshift z >~ 10, by protogalaxies at 6 <~ z <~ 10, or by larger galaxies at 3 <~ z <~ 6. This paper addresses the third possibility by calculating the enrichment of the IGM at z >~ 3 by galaxies of baryonic mass >~ 10^8.5 solar masses. We use already completed cosmological simulations to which we add a prescription for chemical evolution and metal ejection by winds, assuming that the winds have properties similar to those observed in local starbursts and Lyman-break galaxies. Results are given for a number of representative models, and we also examine the properties of the galaxies responsible for the enrichment as well as the physical effects responsible for wind escape and propagation. We find that winds of velocity >~ 200-300 km/s are capable of enriching the IGM to the mean level observed, though many low-density regions would remain metal free. Calibrated by observations of Lyman-break galaxies, our calculations suggest that most galaxies at z >~ 3 should drive winds that can escape and propagate to large radii. The primary effect limiting the enrichment of low-density IG gas in our scenario is then the travel time from high- to low-density regions, implying that the metallicity of low-density gas is a strong function of redshift.