Obscuring Fraction of Active Galactic Nuclei: Implications from Radiation-driven Fountain Models
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 fobs for a given column density toward the AGNs changes depending on both the AGN luminosity and the black hole mass. In particular, fobs for NH ≥ 1022 cm-2 increases from ∼0.2 to ∼0.6 as a function of the X-ray luminosity LX in the LX = 1042-44 erg s-1 range, but fobs becomes small (∼0.4) above a luminosity (∼1045 erg s-1). The behaviors of fobs 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 LAGN, fobs is always smaller (∼0.2-0.3) for a larger column density (NH ≥ 1023 cm-2). We also found cases that more than 70% of the solid angles can be covered by the fountain flows.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2015
- DOI:
- 10.1088/0004-637X/812/1/82
- arXiv:
- arXiv:1509.00576
- Bibcode:
- 2015ApJ...812...82W
- Keywords:
-
- galaxies: active;
- galaxies: nuclei;
- ISM: structure;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 17 pages, 12 figures, appeared in ApJ. corrected typos in Fig. 7 and Fig. 8