Radiation-driven turbulent accretion onto massive black holes
  Accretion of gas and interaction of matter and radiation are at the heart of
many questions pertaining to black hole (BH) growth and coevolution of massive
BHs and their host galaxies. To answer them it is critical to quantify how the
ionizing radiation that emanates from the innermost regions of the BH accretion
flow couples to the surrounding medium and how it regulates the BH fueling. In
this work we use high resolution 3-dimensional (3D) radiation-hydrodynamic
simulations with the code Enzo, equipped with adaptive ray tracing module
Moray, to investigate radiation-regulated BH accretion of cold gas. Our
simulations reproduce findings from an earlier generation of 1D/2D simulations:
the accretion powered UV and X-ray radiation forms a highly ionized bubble,
which leads to suppression of BH accretion rate characterized by quasi-periodic
outbursts. A new feature revealed by the 3D simulations is the highly turbulent
nature of the gas flow in vicinity of the ionization front. During quiescent
periods between accretion outbursts, the ionized bubble shrinks in size and the
gas density that precedes the ionization front increases. Consequently, the 3D
simulations show oscillations in the accretion rate of only ~2-3 orders of
magnitude, significantly smaller than 1D/2D models. We calculate the energy
budget of the gas flow and find that turbulence is the main contributor to the
kinetic energy of the gas but corresponds to less than 10% of its thermal
energy and thus does not contribute significantly to the pressure support of
the gas.
