Radiation-driven Fountain and Origin of Torus around Active Galactic
  Nuclei
  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 +-30 deg and +-50 deg 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 2x10^-4 ~ 10^-3, M_sun/yr 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.
