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, fobs for N-H >= 10(22) cm(-2) increases from similar to 0.2 to similar to 0.6 as a function of the X-ray luminosity L-X in the L-X = 10(42-44) erg s(-1) range, but f(obs) becomes small (similar to 0.4) above a luminosity (similar to 10(45) erg s(-1)). 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 (similar to 0.2-0.3) for a larger column density (N-H >= 10(23) cm(-2)). We also found cases that more than 70% of the solid angles can be covered by the fountain flows.