Rapid growth of high-redshift black holes

We discuss a model for the early assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical buildup far up into the dark halo "merger tree.'' Motivated by the observations of luminous quasars around redshift z approximate to 6 with SMBH masses approximate to 10(9) M., we assess the possibility of an early phase of stable supercritical quasi-spherical accretion in the BHs hosted by metal-free halos with virial temperature T-vir > 10(4) K. We assume that the first "seed'' black holes formed with intermediate masses following the collapse of the first generation of stars in minihalos collapsing at z similar to 20 from high-sigma density fluctuations. In high-redshift halos with T-vir > 10(4) K, conditions exist for the formation of a fat disk of gas at T-gas approximate to 5000-10,000 K. Cooling via hydrogen atomic lines is in fact effective in these comparatively massive halos. The cooling and collapse of an initially spherical configuration of gas leads to a rotationally supported disk at the center of the halo if baryons preserve their specific angular momentum during collapse. The conditions for the formation of the gas disk and accretion onto central black holes out of this supply of gas are investigated, as well as the feedback of the emission onto the host and onto the intergalactic medium. We find that even a short phase of supercritical accretion eases the requirements set by the z approximate to 6 quasars.