The cosmological evolution of metal enrichment in quasar host galaxies

Abstract

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the Lambda-cold dark matter model. Galaxy formation in the simulations is coupled with a prescription for black hole activity enabling us to study the evolution of the metal enrichment in quasar hosts and hence explore the relationship between star/spheroid formation and black hole growth/activity. We find a steep radial metallicity gradient in quasar host galaxies, with gas metallicities close to solar values in the outer parts but becoming supersolar in the center. The hosts of the rare bright quasars at z~5-6 have star formation rates of several hundred solar masses per year and halo masses of order ~10^12 solar masses. Already at these redshifts they have supersolar (Z ~2-3 solar) central metallicities, with a mild dependence of metallicity on luminosity, consistent with observed trends. The mean value of metallicity is sensitive to the assumed quasar lifetime, providing a useful new probe of this parameter. We find that lifetimes from 10^7-4x10^7yr are favored by comparison to observational data. In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activity