Anisotropic Galactic Outflows and Enrichment of the Intergalactic
  Medium. II. Numerical Simulations
  We combine an analytic model for anisotropic outflows and galaxy formation
with numerical simulations of large-scale structure and halo formation to study
the impact of galactic outflows on the evolution of the IGM. We have simulated
the evolution of a comoving volume (15 Mpc)^3 in the LCDM universe. We follow
the formation of 20000-60000 galaxies and simulate the galactic outflows
produced by these galaxies, for five outflow opening angles, alpha=60, 90, 120,
150, and 180 degrees (isotropic outflows). Anisotropic outflows follow the path
of least resistance and thus travel preferentially into low-density regions,
away from cosmological structures where galaxies form. These anisotropic
outflows are less likely to overlap with one another, or to hit pre-galactic
collapsing halos and strip them of their gas, preventing a galaxy from forming.
Going from 180 deg to 60 deg, the number of galaxies that actually form
doubles, producing twice as many outflows, and these outflows overlap to a
lesser extent. As a result, the metal volume filling factor of the IGM goes
from 8% for isotropic outflows up to 28% for anisotropic ones. High density
regions are more efficiently enriched than low density ones (~80% compared to
~20% by volume), even though most enriched regions are low densities.
Increasing the anisotropy of outflows increases the extent of enrichment at all
densities, low and high. This is in part because anisotropic outflows are more
numerous. When this effect is factored-out, we find that the probability a
galaxy will enrich systems at densities up to 10 rho_mean is higher for
increasingly anisotropic outflows. This is an effect of the dynamical evolution
of the IGM. Anisotropic outflows expand preferentially into underdense gas, but
that gas can later accrete onto overdense structures.
