Anisotropic galactic outflows and enrichment of the intergalactic medium. II. Numerical simulations

Abstract

We combine an analytic model for anisotropic outflows and galaxy formation with numerical simulations of largescale structure and halo formation to study the impact of galactic outflows on the evolution of the intergalactic medium (IGM). Using this algorithm, we have simulated the evolution of a comoving volume of size (15 Mpc)3 in the ΛCDM universe. Using an N-body simulation starting at redshift z = 24, we follow the formation of galaxies and simulate the galactic outflows produced by these galaxies. Outflows are modeled as bipolar cones with opening angle α, which expand along the direction of least resistance. We consider five opening angles: α = 60°, 90°, 120°, 150°, and 180° (isotropic outflows).We also consider the effect of photoionization suppression of galaxy formation by reionization at redshift z = 6. Anisotropic outflows travel preferentially into low-density regions, away from cosmological structures (filaments and pancakes) where galaxies form. These anisotropic outflows are less likely to overlap with one another than isotropic ones. They are also less likely to hit pre-galactic collapsing halos and strip them of their gas, preventing a galaxy from forming. Going from 180° to 60°, 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 10ρ̄. is higher for increasingly anisotropic outflows. This is interpreted as 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. The inclusion of photoionization greatly reduces the formation of low-mass galaxies at redshifts z < 3. The result is a decline in the physical extent of galactic outflows after z = 3 as accretion overwhelms the expansion of new outflows and reduces feedback in underdense regions. © 2010 The American Astronomical Society. All rights reserved.