Obscuring Fraction of Active Galactic Nuclei: Implications from
  Radiation-driven Fountain Models
  Active galactic nuclei (AGNs) are believed to be obscured by an optical thick
"torus" that covers a large fraction of solid angles for the nuclei. However,
the physical origin of the tori and the differences in the tori among AGNs are
not clear. In a previous paper based on three-dimensional
radiation-hydorodynamic calculations, we proposed a physics-based mechanism for
the obscuration, called "radiation-driven fountains," in which the circulation
of the gas driven by central radiation naturally forms a thick disk that
partially obscures the nuclear emission. Here, we expand this mechanism and
conduct a series of simulations to explore how obscuration depends on the
properties of AGNs. We found that the obscuring fraction f_obs for a given
column density toward the AGNs changes depending on both the AGN luminosity and
the black hole mass. In particular, f_obs for N_H \geq 10^22 cm^-2 increases
from ~0.2 to ~0.6 as a function of the X-ray luminosity L_X in the 10^{42-44}
ergs/s range, but f_obs becomes small (~0.4) above a luminosity (~10^{45}
ergs/s). The behaviors of f_obs can be understood by a simple analytic model
and provide insight into the redshift evolution of the obscuration. The
simulations also show that for a given L_AGN, f_obs is always smaller
(~0.2-0.3) for a larger column density (N_H \geq 10^23 cm^-2). We also found
cases that more than 70% of the solid angles can be covered by the fountain
flows.
