Formation ofz∼6 Quasars from Hierarchical Galaxy Mergers

The discovery of luminous quasars at redshift z ~ 6 indicates the presence ofsupermassive black holes (SMBHs) of mass ~10^9 Msun when the Universe was lessthan one billion years old. This finding presents several challenges fortheoretical models. Here, we present the first multi-scale simulations that,together with a self-regulated model for the SMBH growth, produce a luminousquasar at z ~ 6.5 in the LCDM paradigm. We follow the hierarchical assemblyhistory of the most massive halo in a ~ 3 Gpc^3 volume, and find that this haloof ~ 8x 10^{12} Msun forming at z ~ 6.5 after several major mergers is able toreproduce a number of observed properties of SDSS J1148+5251, the most distantquasar detected at z =6.42 (Fan et al. 2003). Moreover, the SMBHs grow throughgas accretion below the Eddington limit in a self-regulated manner owing tofeedback. We find that the progenitors experience significant star formation(up to 10^4 Msun/yr) preceding the major quasar phase such that the stellarmass of the quasar host reaches 10^{12} Msun at z ~ 6.5, consistent withobservations of significant metal enrichment in SDSS J1148+5251. Our resultsprovide a viable formation mechanism for z ~ 6 quasars in the standard LCDMcosmology, and demonstrate a common, merger-driven origin for the rarestquasars and the fundamental SMBH-host correlation in a hierarchicalUniverse.(Abridged)