Anisotropic AGN Outflows and Enrichment of the Intergalactic Medium. II.
  Metallicity
  We investigate the large-scale influence of outflows from AGNs in enriching
the IGM with metals in a cosmological context. We combine cosmological
simulations of large scale structure formation with a detailed model of metal
enrichment, in which outflows expand anisotropically along the direction of
least resistance, distributing metals into the IGM. The metals carried by the
outflows are generated by two separate stellar populations: stars located near
the central AGN, and stars located in the greater galaxy. Using this algorithm,
we performed a series of 5 simulations of the propagation of AGN-driven
outflows in a cosmological volume of size (128/h Mpc)^3 in a Lambda-CDM
universe, and analyze the resulting metal enrichment of the IGM. We found that
the metallicity induced in the IGM is greatly dominated by AGNs having
bolometric luminosity L > 10^9 L_sun, sources with 10^8 < L / L_sun < 10^9
having a negligible contribution. Our simulations produced an average IGM
metallicity of [O/H] = -5 at z = 5.5, which then rises gradually, and remains
relatively flat at a value [O/H] = -2.8 between z = 2 and z = 0. The ejection
of metals from AGN host galaxies by AGN-driven outflows is found to enrich the
IGM to > 10 - 20% of the observed values, the number dependent on redshift. The
enriched IGM volume fractions are small at z > 3, then rise rapidly to the
following values at z = 0: 6 - 10% of the volume enriched to [O/H] > -2.5, 14 -
24% volume to [O/H] > -3, and 34 - 45% volume to [O/H] > -4. At z > 2, there is
a gradient of the induced enrichment, the metallicity decreasing with
increasing IGM density, enriching the underdense IGM to higher metallicities, a
trend more prominent with increasing anisotropy of the outflows. This can
explain observations of metal-enriched low-density IGM at z = 3 - 4.
