Abstract
We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the ?CDM 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 ~1012 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 107 to 4 ? 107 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.
Highlights
The presence of supermassive black holes in the centers of nearby galaxies with a significant spheroidal component supports arguments that there is a fundamental link between the assembly of black holes and the formation of spheroids in galaxy halos
The evidence indicates that the mass of the central black hole is correlated with the bulge luminosity (e.g., Magorrian et al 1998; Kormendy & Gebhardt 2001) and even more tightly with the velocity dispersion of its host bulge (Tremaine et al 2002; Ferrarese & Merritt 2000; Merritt & Ferrarese 2001; Gebhardt et al 2000), implying that the process that leads to the formation of galactic spheroids must be intimately linked to the growth of central supermassive black holes with commensurate mass
In an earlier paper (Di Matteo et al 2003), we assumed that the black hole fueling rate is regulated by star formation in the gas. We showed that this simple assumption can explain the observed black hole mass and spheroid velocity dispersion relation and the broad properties of the quasar luminosity functions. It leads to a one-to-one relation between black hole accretion rate density evolution and the star formation history, which at high redshift implies that the quasar phase and star formation rates both grow in response to the growth of the halo mass function
Summary
The presence of supermassive black holes in the centers of nearby galaxies with a significant spheroidal component supports arguments that there is a fundamental link between the assembly of black holes and the formation of spheroids in galaxy halos. We use cosmological hydrodynamical simulations coupled with a prescription for black hole activity in galaxies to follow the evolution of the metal enrichment in quasar host galaxies and explore the relation between star/spheroid formation and black hole growth/activity. In an earlier paper (Di Matteo et al 2003), we assumed that the black hole fueling rate is regulated by star formation in the gas We showed that this simple assumption can explain the observed black hole mass and spheroid velocity dispersion relation and the broad properties of the quasar luminosity functions (for an assumed quasar lifetime). It leads to a one-to-one relation between black hole accretion rate density evolution and the star formation history, which at high redshift implies that the quasar phase and star formation rates both grow in response to the growth of the halo mass function.
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