The effect of variations in the input physics on the cosmic distribution of metals predicted by simulations

Abstract

[Abridged] We investigate how a range of physical processes affect the cosmic metal distribution using a suite of cosmological, hydrodynamical simulations. Focusing on z = 0 and 2, we study the metallicities and metal mass fractions for stars as well as for the ISM, and several more diffuse gas phases. We vary the cooling rates, star formation law, structure of the ISM, galactic winds, feedback from AGN, reionization history, stellar IMF, and cosmology. In all models stars and the warm-hot IGM (WHIM) constitute the dominant repository of metals, while for z > 2 the ISM is also important. In models with galactic winds, predictions for the metallicities of the various phases vary at the factor of two level and are broadly consistent with observations. The exception is the cold-warm IGM, whose metallicity varies at the order of magnitude level if the prescription for galactic winds is varied, even for a fixed wind energy per unit stellar mass formed, and falls far below the observed values if winds are not included. At the other extreme, the metallicity of the intracluster medium (ICM) is largely insensitive to the presence of galactic winds, indicating that its enrichment is regulated by other processes. The mean metallicities of stars (~ Z_sun), the ICM (~ 0.1 Z_sun), and the WHIM (~ 0.1 Z_sun) evolve only slowly, while those of the cold halo gas and the IGM increase by more than an order of magnitude from z = 5 to 0. Higher velocity outflows are more efficient at transporting metals to low densities, but actually predict lower metallicities for the cold-warm IGM since the winds shock-heat the gas to high temperatures, thereby increasing the fraction of the metals residing in, but not the metallicity of, the WHIM. Besides galactic winds driven by feedback from star formation, the metal distribution is most sensitive to the inclusion of metal-line cooling and feedback from AGN.