Cosmological Simulations of Intergalactic Medium Enrichment from Galactic Outflows

Abstract

We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z=6 to 1.5, based on hydrodynamic simulations of structure formation that incorporate galactic outflows. Our main result is that outflow parameterizations based on momentum-driven winds as seen in local starburst galaxies provide the best agreement with observations of CIV absorption at z~2-5. Such models sufficiently enrich the high-z IGM to produce a global mass density of CIV absorbers that is relatively invariant from z=5.5 to 1.5, in agreement with observations. This occurs despite an increase in the volume-averaged metallicity by x5-10 over this redshift range, because energy input from outflows causes a drop in the global ionization fraction of CIV. Comparisons to observed CIV column density and linewidth distributions and CIV-based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few % with volume-weighted metallicities around 0.001 at z~3, significant amounts of collisionally-ionized CIV absorption, and a metallicity-density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally-calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time.