The cosmological evolution of metal enrichment in quasar host galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the LambdaCDM 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 similar to 5-6 have star formation rates of several hundred M-circle dot yr(-1) and halo masses of order similar to10(12) M-circle dot. Already at these redshifts they have supersolar (Z/Z(circle dot) similar to 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 x 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 similar to 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(circle dot) degrees 0.4-1) as we approach the epoch of the first significant star formation activity.