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 build-up far up in the dark halo merger tree'. Motivated by the observations of luminous quasars around redshift z=6 with SMBH masses of billion solar masses, we assess the possibility of an early phase of stable super-critical quasi-spherical accretion in the BHs hosted by metal free halos with virial temperature larger than 10000 K. We assume that the first seed’ black holes formed with intermediate masses following the collapse of the first generation of stars, in mini-halos collapsing at z=20-30 from high peaks of density fluctuations. In high redshift halos with virial temperature larger than 10000 K, conditions exist for the formation of a fat disc of gas at T_gas=5000-10000 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 disc at the center of the halo if baryons preserve their specific angular momentum during collapse. The conditions for the formation of the gas disc and accretion onto a 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=6 quasars.