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The discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~10⁹ Msun when the Universe was less than one billion years old. This finding presents several challenges for theoretical models. Here, we present the first multi-scale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6. 5 in the LCDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~ 3 Gpc³ volume, and find that this halo of ~ 8x 10^12 Msun forming at z ~ 6. 5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z =6. 42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience significant star formation (up to 10⁴ Msun/yr) preceding the major quasar phase such that the stellar mass of the quasar host reaches 10^12 Msun at z ~ 6. 5, consistent with observations of significant metal enrichment in SDSS J1148+5251. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard LCDM cosmology, and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental SMBH-host correlation in a hierarchical Universe. (Abridged)
Li et al. (Fri,) studied this question.