The Cosmological Evolution of Metal Enrichment in Quasar Host Galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the ΛCDM 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. In order to assess effects of numerical resolution, we compare simulations with different particle numbers and box sizes. 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 M ☉ yr -1 and halo masses of order ~10 12 M ☉ . Already at these redshifts they have supersolar ( Z / Z ☉ ~ 2-3) 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 to 4 × 10 7 yr are favored by comparison with 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 toward solar and slightly subsolar values ( Z / Z ☉ ~ 0.4-1) as we approach the epoch of the first significant star formation activity.