The Cosmological Evolution of Metal Enrichment in Quasar Host Galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamicsimulations of the Lambda-cold dark matter model. Galaxy formation in thesimulations is coupled with a prescription for black hole activity enabling usto study the evolution of the metal enrichment in quasar hosts and henceexplore the relationship between star/spheroid formation and black holegrowth/activity. We find a steep radial metallicity gradient in quasar hostgalaxies, with gas metallicities close to solar values in the outer parts butbecoming supersolar in the center. The hosts of the rare bright quasars atz~5-6 have star formation rates of several hundred solar masses per year andhalo masses of order ~10^12 solar masses. Already at these redshifts they havesupersolar (Z ~2-3 solar) central metallicities, with a mild dependence ofmetallicity on luminosity, consistent with observed trends. The mean value ofmetallicity is sensitive to the assumed quasar lifetime, providing a useful newprobe of this parameter. We find that lifetimes from 10^7-4x10^7yr are favoredby comparison to observational data. In both the models and observations, therate of evolution of the mean quasar metallicity as a function of redshift isgenerally flat out to z ~ 4-5. Beyond the observed redshift range and out toredshift z ~ 6-8, we predict a slow decline of the mean central metallicitytowards solar and slightly subsolar values as we approach the epoch of thefirst significant star formation activityComment: submitted to ApJ, 13 pages, 9 figures, emulateapj.sty, corrected typos and added referenc