We propose a plausible mechanism to explain the formation of the so-called “ obscuring tori ” around active galactic nuclei ( AGNs ) based on three-dimensional hydrodynamic simulations including radiative feedback from the central source .
The X-ray heating and radiation pressure on the gas are explicitly calculated using a ray-tracing method .
This radiation feedback drives a “ fountain ” , that is , a vertical circulation of gas in the central a few to tens parsecs .
Interaction between the non-steady outflows and inflows causes the formation of a geometrically thick torus with internal turbulent motion .
As a result , the AGN is obscured for a wide range of solid angles .
In a quasi-steady state , the opening angles for the column density toward a black hole < 10 ^ { 23 } cm ^ { -2 } are approximately \pm 30 ^ { \circ } and \pm 50 ^ { \circ } for AGNs with 10 % and 1 % Eddington luminosity , respectively .
Mass inflows through the torus coexist with the outflow and internal turbulent motion , and the average mass accretion rate to the central parsec region is 2 \times 10 ^ { -4 } \sim 10 ^ { -3 } M _ { \odot } { yr } ^ { -1 } ; this is about ten times smaller than accretion rate required to maintain the AGN luminosity .
This implies that relatively luminous AGN activity is intrinsically intermittent or that there are other mechanisms , such as stellar energy feedback , that enhance the mass accretion to the center .