2016MNRAS.458.3047P

The assembly of the first super massive black holes (SMBHs) at z >= 6 is still a subject of intense debate. If black holes (BHs) grow at their Eddington rate, they must start from >=10⁴ M_☉ seeds formed by the direct collapse of gas. Here, we explore the alternative scenario where ∼100 M_☉ BH remnants of the first stars grow at super-Eddington rate via radiatively inefficient slim accretion discs. We use an improved version of the cosmological, data-constrained semi-analytic model GAMETE/QSOdust, where we follow the evolution of nuclear BHs and gas cooling, disc and bulge formation of their host galaxies. Adopting SDSS J1148+5251 (J1148) at z=6.4 as a prototype of luminous z >= 6 quasars, we find that ∼80 per cent of its SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ∼ 10 in dense, gas-rich environments. The average BH mass at z ∼ 20 is M_BH >= 10⁴ M_☉, comparable to that of direct collapse BHs. At z=6.4 the AGN-driven mass outflow rate is consistent with the observations and the BH-to-bulge mass ratio is compatible with the local scaling relation. However, the stellar mass in the central 2.5 kpc is closer to the value inferred from CO observations. Finally, ∼20 per cent of J1148 progenitors at z=7.1 have BH luminosities and masses comparable to ULAS J1120+0641, suggesting that this quasar may be one of the progenitors of J1148.