Supermassive Black Hole in an Elliptical Galaxy: Accretion of a Hot Gas with a Low but Finite Angular Momentum

Abstract

The accretion of hot slowly rotating gas onto a supermassive black hole is considered. Rotation velocities at the Bondi radius r_B are small in comparison with speed of sound c_s. The centrifugal barrier at a depth r_c = l^2/G M_BH << r_B hinders supersonic accretion. We take into account saturated electron thermal conductivity and Bremsstrahlung energy losses of two temperature plasma for density and temperature near the Bondi radius similar to those observed in M87 galaxy. Joint action of electron thermal conductivity and free-free radiation leads to the effective cooling of accreting plasma and formation of the subsonic settling of accreting gas above the zone of a centrifugal barrier. A toroidal condensation and a hollow funnel that separates the torus from the black hole emerge near the barrier. The barrier divides the flow into two regions: (1) the settling zone with slow subKeplerian rotation and (2) the zone with rapid supersonic nearly Keplerian rotation. Existence of the centrifugal barrier leads to significant decrease of the accretion rate dM/dt in comparison with the critical Bondi solution for gamma=5/3. Shear instabilities in the torus and related friction cause the gas to spread slowly in the equatorial plane in two directions. As a result, outer (r>r_c) and inner (r<r_c) disks are formed. The gas enters the zone of the internal ADAF flow along the accretion disk (r<r_c). Since the angular momentum is conserved, the outer disk removes outward an excess of angular momentum along with part of the matter falling into the torus. Such outer Keplerian disk was observed by Hubble Space Telescope around the nucleus of the M87 galaxy in the optical emission lines. Turbulence causes rotation. We discuss the characteristic times during which the turbulence should lead to the changes in the orientation of the torus, accretion disk and, possibly, of the jet.