Multiscale mass transport in z ∼6 galactic discs: fuelling black holes

Abstract

By using Adaptive Mesh Refinement cosmological hydrodynamic N-body zoom-in simulations, with the RAMSES code, we studied the mass transport processes on to galactic nuclei from high redshift up to z ∼6. Due to the large dynamical range of the simulations, we were able to study the mass accretion process on scales from ∼50 kpc to ∼few 1 pc. We studied the black hole (BH) growth on to the Galactic Centre in relation with the mass transport processes associated to both the Reynolds stress and the gravitational stress on the disc. Such methodology allowed us to identify the main mass transport process as a function of the scales of the problem. We found that in simulations that include radiative cooling and supernovae feedback, the supermassive black hole (SMBH) grows at the Eddington limit for some periods of time presenting fEDD ≈ 0.5 throughout its evolution. The α parameter is dominated by the Reynolds term, αR, with αR 1. The gravitational part of the α parameter, αG, has an increasing trend towards the Galactic Centre at higher redshifts, with values αG ∼1 at radii few 101 pc contributing to the BH fuelling. In terms of torques, we also found that gravity has an increasing contribution towards the Galactic Centre at earlier epochs with a mixed contribution above ∼100 pc. This complementary work between pressure gradients and gravitational potential gradients allows an efficient mass transport on the disc with average mass accretion rates of the order of ∼few 1 M yr−1. These levels of SMBH accretion rates found in our cosmological simulations are needed in all models of SMBH growth that attempt to explain the formation of redshift 6–7 quasars.