  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.
