ACTIVE GALACTIC NUCLEUS OBSCURATION THROUGH DUSTY INFRARED-DOMINATED FLOWS. I. RADIATION-HYDRODYNAMICS SOLUTION FOR THE WIND

We construct a radiation-hydrodynamics model for the obscuring toroidal structure in active galactic nuclei. In this model the obscuration is produced at parsec scales by a dense, dusty wind which is supported by infrared radiation pressure on dust grains. To find the distribution of radiation pressure, we numerically solve the two-dimensional radiation transfer problem in a flux-limited diffusion approximation. We iteratively couple the solution with calculations of stationary one-dimensional models for the wind and obtain the z-component of the velocity. Our results demonstrate that for active galactic nucleus (AGN) luminosities greater than 0.1 L-edd, external illumination can support a geometrically thick obscuration via outflows driven by infrared radiation pressure. The terminal velocity of marginally Compton-thin models (0.2 < tau(T) < 0.6) is comparable to or greater than the escape velocity. In Compton-thick models the maximum value of the vertical component of the velocity is lower than the escape velocity, suggesting that a significant part of our torus is in the form of failed wind. The results demonstrate that obscuration via normal or failed infrared-driven winds is a viable option for the AGN torus problem and AGN unification models. Such winds can also provide an important channel for AGN feedback.