Low density, radiatively inefficient rotating-accretion flow onto a
  black hole
  We study low-density axisymmetric accretion flows onto black holes (BHs) with
two-dimensional hydrodynamical simulations, adopting the $\alpha$-viscosity
prescription. When the gas angular momentum is low enough to form a
rotationally supported disk within the Bondi radius ($R_{\rm B}$), we find a
global steady accretion solution. The solution consists of a rotational
equilibrium distribution at $r\sim R_{\rm B}$, where the density follows $\rho
\propto (1+R_{\rm B}/r)^{3/2}$, surrounding a geometrically thick and optically
thin accretion disk at the centrifugal radius, where thermal energy generated
by viscosity is transported via strong convection. Physical properties of the
inner solution agree with those expected in convection-dominated accretion
flows (CDAF; $\rho \propto r^{-1/2}$). In the inner CDAF solution, the gas
inflow rate decreases towards the center due to convection ($\dot{M}\propto
r$), and the net accretion rate (including both inflows and outflows) is
strongly suppressed by several orders of magnitude from the Bondi accretion
rate $\dot{M}_{\rm B}$ The net accretion rate depends on the viscous strength,
following $\dot{M}/\dot{M}_{\rm B}\propto (\alpha/0.01)^{0.6}$. This solution
holds for low accretion rates of $\dot{M}_{\rm B}/\dot{M}_{\rm Edd}< 10^{-3}$
having minimal radiation cooling, where $\dot{M}_{\rm Edd}$ is the Eddington
rate. In a hot plasma at the bottom ($r<10^{-3}~R_{\rm B}$), thermal conduction
would dominate the convective energy flux. Since suppression of the accretion
by convection ceases, the final BH feeding rate is found to be
$\dot{M}/\dot{M}_{\rm B} \sim 10^{-3}-10^{-2}$. This rate is as low as
$\dot{M}/\dot{M}_{\rm Edd} \sim 10^{-7}-10^{-6}$ inferred for SgrA$^*$ and the
nuclear BHs in M31 and M87, and can explain the low luminosities in these
sources, without invoking any feedback mechanism.
