Radiation-driven Fountain and Origin of Torus around Active Galactic Nuclei

TL;DR

This paper presents a hydrodynamic simulation-based model explaining the formation of obscuring tori around AGNs through radiation-driven gas fountains, revealing their structure, dynamics, and implications for AGN obscuration and activity.

Contribution

It introduces a novel radiation feedback mechanism causing torus formation via gas fountains, incorporating detailed radiative transfer and turbulence effects in three-dimensional simulations.

Findings

Formation of a geometrically thick, turbulent torus around AGNs.

Obscuration angles depend on Eddington ratio, approximately +-30 to +-50 degrees.

Mass inflow rates are insufficient for continuous AGN luminosity, suggesting intermittency or additional mechanisms.

Abstract

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.