Radiation-driven, geometrically thick, dusty obscuration in active galactic nuclei
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Date
2016-10-05
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Johns Hopkins University
Abstract
Substantial evidence points to dusty, geometrically thick tori obscuring the central engines of active galactic nuclei (AGNs), but so far no mechanism satisfactorily explains why cool dust in the torus remains in a puffy geometry. Near-Eddington infrared (IR) and ultraviolet (UV) luminosities coupled with high dust opacities at these frequencies suggest that radiation pressure on dust can play a significant role in shaping the torus. Here we explore this possibility with three-dimensional radiative magnetohydrodynamics simulations. Our code simultaneously solves the hydrodynamics equations, the time-dependent multi–angle group IR radiative transfer (RT) equation, and the time-independent UV RT equation. We find a highly dynamic situation. IR radiation is anisotropic, leaving primarily through the central hole. Since IR and UV radiative accelerations increase with latitude, our torus naturally settles into a steady state with inflow along the mid-plane and outflow near the inner surface. The covering fraction and column density distribution of our torus are stable over time and roughly agree with observations. The outflow has speed and mass loss rate close to observed values. Most importantly, our simulations demonstrate that isolated tori cannot exist indefinitely because outflow removes mass and radiative acceleration removes binding energy; this means realistic tori are determined by the rate of mass resupply from galactic scales, as well as stresses both internal to the tori and in the outflow.
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galaxies: active, galaxies: nuclei, quasars: general, methods: numerical, hydrodynamics, radiative transfer