Parsec-scale accretion and winds irradiated by a quasar

Abstract

We present numerical simulations of properties of a parsec-scale torus exposed to illumination by the central black hole in an active galaxy (AGN). Our physical model allows to investigate the balance between the formation of winds and accretion simultaneously. Radiation-driven winds are allowed by taking into account radiation pressure due to UV and IR radiation along with X-ray heating and dust sublimation. Accretion is allowed through angular momentum transport and the solution of the equations of radiation hydrodynamics. Our methods adopt flux-limited diffusion radiation-hydrodynamics for the dusty, infrared pressure driven part of the flow, along with X-ray heating and cooling. Angular momentum transport in the accreting part of the flow is modeled using effective viscosity. Our results demonstrate that radiation pressure on dust can play an important role in shaping AGN obscuration. For example, when the luminosity illuminating the torus exceeds $L>0.01\,L_{\rm Edd}$, where $L_{\rm Edd}$ is the Eddington luminosity, we find no episodes of sustained disk accretion because radiation pressure does not allow a disk to form. Despite the absence of the disk accretion, the flow of gas to smaller radii still proceeds at a rate $10^{-4}-10^{-1}\,M_\odot\,{\rm yr}^{-1}$ through the capturing of the gas from the hot evaporative flow, thus providing a mechanism to deliver gas from a radiation-pressure dominated torus to the inner accretion disk. As $L/L_{\rm edd}$ increases, larger radiation input leads to larger torus aspect ratios and increased obscuration of the central black hole. We also find the important role of the X-ray heated gas in shaping of the obscuring torus.