Metal Enrichment of the Intergalactic Medium in Cosmological Simulations

Abstract

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~0.1-1% solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to 1/3-1/2 solar metallicity at z=0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressure driven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations, we add dust and (gaseous) metals, distributing the dust and metals in the gas according to three simple parameterized prescriptions, one for each enrichment mechanism. These prescriptions are formulated to capture the basic ejection physics, and calibrated when possible with empirical data. Our results indicate that dynamical removal of metals from >~ 3*10^8 solar mass galaxies cannot account for the observed metallicity of low-column density Ly-alpha absorbers, and that dynamical removal from >~ 3*10^10 solar mass galaxies cannot account for the ICM metallicities. Dynamical removal also fails to produce a strong enough mass-metallicity relation in galaxies. In contrast, either wind or radiation-pressure ejection of metals from relatively large galaxies can plausibly account for all three sets of observations (though it is unclear whether metals can be distributed uniformly enough in the low-density regions without overly disturbing the IGM, and whether clusters can be enriched quite as much as observed). We investigate in detail how our results change with variations in our assumed parameters, and how results for the different ejection processes compare. (Abridged)