Radiation-driven Feedback to the ISM around AGNs

Abstract

Although the “donut-like” obscuring molecular torus is often postulated to explain the type-1 and -2 dichotomy in AGNs, its physical origin is still unclear. We propose a plausible mechanism to explain the formation of the geometrically and optically thick torus, i.e. radiation-driven fountain. Using 3-D hydrodynamic simulations including radiative feedback from the central source, taking into account the X-ray heating and radiation pressure on the gas, we found that a vertical circulation of gas is generated in the central 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 < 10²³ cm⁻² are approximately ± 30° and ± 50° for AGNs with 10% and 1% Eddington luminosity, respectively. Mass inflows through the torus coexist with the outflow and internal turbulent motion, although the average mass accretion rate to the central parsec region is about ten times smaller than the accretion rate required to maintain the assumed 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.