Dust Dynamics in AGN Winds: A New Mechanism For Multiwavelength AGN Variability

Abstract: Partial dust obscuration in active galactic nuclei (AGN) has been proposed as a potential explanation for some cases of AGN variability. The dust-gas mixture present in AGN tori is accelerated by radiation pressure, leading to the launching of an AGN wind. Dust under these conditions has been shown to be unstable to a generic class of fast-growing resonant drag instabilities (RDIs). In this work, we present the first numerical simulations of radiation-driven outflows that explicitly include dust dynamics in conditions resembling AGN winds. We investigate the implications of RDIs on the torus morphology, AGN variability, and the ability of radiation to effectively launch a wind. We find that the RDIs rapidly develop, reaching saturation at times much shorter than the global timescales of the outflows, resulting in the formation of filamentary structure on box-size scales with strong dust clumping and super-Alfv\'enic velocity dispersions. The instabilities lead to fluctuations in dust opacity and gas column density of 10-20\% when integrated along mock observed lines-of-sight to the quasar accretion disk. These fluctuations occur over year to decade timescales and exhibit a red-noise power spectrum commonly observed for AGN. Additionally, we find that the radiation effectively couples with the dust-gas mixture, launching highly supersonic winds that entrain 70-90\% of the gas, with a factor of $\lesssim 3$ photon momentum loss relative to the predicted multiple-scattering momentum loading rate. Therefore, our findings suggest that RDIs play an important role in driving the clumpy nature of AGN tori and generating AGN variability consistent with observations.