Rapid Black Hole Growth under Anisotropic Radiation Feedback
  Discovery of high-redshift (z > 6) supermassive black holes (BHs) may
indicate that the rapid (or super-Eddington) gas accretion has aided their
quick growth. Here, we study such rapid accretion of the primordial gas on to
intermediate-mass (10^2 - 10^5 M_sun) BHs under anisotropic radiation feedback.
We perform two-dimensional radiation hydrodynamics simulations that solve the
flow structure across the Bondi radius, from far outside of the Bondi radius
down to a central part which is larger than a circum-BH accretion disc. The
radiation from the unresolved circum-BH disc is analytically modeled
considering self-shadowing effect. We show that the flow settles into a steady
state, where the flow structure consists of two distinct parts: (1) bipolar
ionized outflowing regions, where the gas is pushed outward by thermal gas
pressure and super-Eddington radiation pressure, and (2) an equatorial neutral
inflowing region, where the gas falls toward the central BH without affected by
radiation feedback. The resulting accretion rate is much higher than that in
the case of isotropic radiation, far exceeding the Eddington-limited rate to
reach a value slightly lower than the Bondi one. The opening angle of the
equatorial inflowing region is determined by the luminosity and directional
dependence of the central radiation. We find that photoevaporation from its
surfaces set the critical opening angle of about ten degrees below which the
accretion to the BH is quenched. We suggest that the shadowing effect allows
even stellar-remnant BHs to grow rapidly enough to become high-redshift
supermassive BHs.
