Formation of High-redshift (z>6) Quasars Driven by Nuclear Starbursts

Based on the physical model of a supermassive black hole (SMBH) growth via gas accretion in a circumnuclear disk (CND) proposed by Kawakatu & Wada (2008), we describe the formation of high-$z$ ($z > 6$) quasars (QSOs) whose BH masses are M_{BH}> 10^{9} M_{\odot}. We derive the necessary conditions to form QSOs at z > 6 by only gas accretion: (i) A large mass supply with M_{sup} > 10^{10}M_{\odot} from host galaxies to CNDs, because the final BH mass is only 1-10% of the total supplied mass from QSO hosts. (ii) High star formation efficiency for a rapid BH growth. We also find that if the BH growth is limited by the Eddington accretion, the final BH mass is greatly suppressed. Thus, the super-Eddington growth is required for the QSO formation. The evolution of the QSO luminosity depends on the redshift z_{i} at which accretion onto a seed BH is initiated. In other words, the brighter QSOs at z >6 favor the late growth of SMBHs (i.e., z_{i}=10) rather than early growth (i.e., z_{i}=30). Moreover, we predict the observable properties and the evolution of QSOs at z >6. In a QSO phase, there should exist a stellar rich massive CND, whose gas mass is about 10% of the dynamical mass inside 0.1-1 kpc}. On the other hand, in a phase where the BH grows (i.e., a proto-QSO phase), the proto-QSO has a gas rich massive CNDs whose gas mass is comparable to the dynamical mass (abridged).