SIMBAD references

We study low-density axisymmetric accretion flows on to black holes (BHs) with two-dimensional hydrodynamical simulations, adopting the α-viscosity prescription. When the gas angular momentum is low enough to form a rotationally supported disc within the Bondi radius (R_B), we find a global steady accretion solution. The solution consists of a rotational equilibrium distribution around r ∼ R_B, where the density follows ρ ∝ (1 + R_B/r)^3/2, surrounding a geometrically thick and optically thin accretion disc at the centrifugal radius R_C(<R_B), where thermal energy generated by viscosity is transported via convection. Physical properties of the inner solution agree with those expected in convection-dominated accretion flows (ρ ∝ r^–1/2). In the inner solution, the gas inflow rate decreases towards the centre due to convection ({dot}M∝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_ B_. The net accretion rate depends on the viscous strength, following {dot}M/{dot}M_ B_∝(α/0.01)^0.6. This solution holds for low accretion rates of {dot}M_ B_/{dot}M_ Edd_≲10^–3 having minimal radiation cooling, where {dot}M_ Edd_ is the Eddington accretion rate. In a hot plasma at the bottom (r < 10^–3 R_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_ B_∼10^–3-10^–2. This rate is as low as {dot}M/{dot}M_ Edd_∼10^–7-10^–6 inferred for SgrA* and the nuclear BHs in M31 and M87, and can explain their low luminosities, without invoking any feedback mechanism.