Semi–Analytic Simulations of Galactic Winds: Volume Filling Factor, Ejection of Metals and Parameter Study

We present a semi–analytic treatment of galactic winds within high resolution, large scale cosmological N–body simulations of a ΛCDM Universe. The evolution of winds is investigated by following the expansion of supernova driven supershells around the several hundred thousand galaxies that form in an approximately spherical region of space with diameter 52h−1 Mpc and mean density close to the mean density of the Universe. We focus our attention on the impact of winds on the diffuse intergalactic medium. Initial conditions for mass loss at the base of winds are taken from Shu, Mo & Mao (2003). Results are presented for the volume filling factor and the mass fraction of IGM in winds and their dependence on the model parameters is carefully investigated. We find that a high volume filling factor does not necessarily correspond to a high mass fraction in wind shells, implying that even very spatially extended galactic winds may not leave detectable imprints on the Lyα forest. Low mass galaxies play a major role in seeding the IGM with metals at high redshift in models where winds sweep up little gas from the IGM and supernova ejecta constitute most of the mass in shells. The formation of winds in low mass galaxies is instead suppressed in models in which the mass of IGM entrained in winds is significant. In these models, the IGM is enriched at later times by galaxies with large stellar masses.