Simulations of Cosmic Chemical Enrichment

Abstract

Using a new numerical model for cosmic chemical evolution, we study the influence of hypernova feedback on the star formation and metal enrichment history of the universe. For assumptions which produce plausible results in idealized collapse models of individual galaxies, our cosmological simulations of the standard Λ cold dark matter (CDM) cosmology show a peak of the cosmic star formation rate at z∼ 4, with ∼10 per cent of the baryons turning into stars. We find that the majority of stars in present-day massive galaxies formed in much smaller galaxies at high redshifts, giving them a mean stellar age as old as 10 Gyr, despite their late assembly times. The hypernova feedback drives galactic outflows efficiently in low-mass galaxies, and these winds eject heavy elements into the intergalactic medium. The ejected baryon fraction is larger for less massive galaxies, correlates well with stellar metallicity and amounts to ∼20 per cent of all baryons in total. The resulting enrichment history is broadly consistent with the observed abundances of Lyman break galaxies, of damped Lyman α systems, and of the intergalactic medium. The metallicity of the cold gas in galaxies increases with galaxy mass, which is comparable to observations with a significant scatter. The stellar mass–metallicity relation of the observed galaxy population is well reproduced by the simulation model as a result of the mass-dependent galactic winds. However, star formation does not terminate in massive galaxies at late times in our model, and too few dwarf galaxies are still forming stars. These problems may be due to a lack of resolution, to inappropriate modelling of supernova feedback or to a neglect of other feedback processes such as active galactic nuclei.