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 Lambda cold dark matter (ACDM) universe. The evolution of winds is investigated by following the expansion of supernova-driven superbubbles 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. Results are presented for the volume filling factor and the mass fraction of the intergalactic medium (IGM) affected by winds, and their dependence on the model parameters is carefully investigated. The mass-loading efficiency of bubbles is a key factor to determine the evolution of winds and their global impact on the IGM: the higher the mass loading, the later the IGM is enriched with metals. Galaxies with 10(9) < M-* < 10(10) M-circle dot are responsible for most of the metals ejected into the IGM at z = 3, while galaxies with M-* < 10(9) M-circle dot give a non-negligible contribution only at higher redshifts, when larger galaxies have not yet assembled. We find a higher mean IGM metallicity than Ly alpha forest observations suggest, and we argue that the discrepancy may be explained by the high temperatures of a large fraction of the metals in winds, which may not leave detectable imprints in absorption in the Lya forest.