Obscuring Fraction of Active Galactic Nuclei: Implications from Radiation-driven Fountain Models

TL;DR

This paper investigates the physical origins of obscuring tori in active galactic nuclei using radiation-driven fountain models, revealing how obscuration depends on AGN luminosity and black hole mass, with implications for understanding AGN evolution.

Contribution

It expands previous radiation-hydrodynamic models to explore how AGN obscuration varies with luminosity and black hole mass, providing a physics-based explanation for observed obscuration fractions.

Findings

Obscuring fraction increases with X-ray luminosity up to ~10^{44} ergs/s.

Higher column densities correspond to smaller obscuration fractions.

Fountain flows can cover more than 70% of solid angles around AGNs.

Abstract

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.