Dynamical complexity in microscale disk-wind systems

Abstract

Context. Powerful winds at accretion-disk scales have been observed in the past 20 years in many active galactic nuclei (AGN). These are the so-called ultrafast outflows (UFOs). Outflows are intimately related to mass accretion through the conservation of angular momentum, and they are therefore a key ingredient of most accretion disk models around black holes (BHs). At the same time, nuclear winds and outflows can provide the feedback that regulates the joint BH and galaxy growth. Aims. We reconsidered UFO observations in the framework of disk-wind scenarios, both magnetohydrodynamic disk winds and radiatively driven winds. Methods. We studied the statistical properties of observed UFOs from the literature and derived the distribution functions of the ratio ω̄ of the mass-outflow and -inflow rates and the ratio λw of the mass-outflow and the Eddington accretion rates. We studied the links between ω̄ and λw and the Eddington ratio λ = Lbol/LEdd. We derived the typical wind-activity history in our sources by assuming that it can be statistically described by population functions. Results. We find that the distribution functions of ω̄ and λw can be described as power laws above some thresholds, suggesting that there may be many wind subevents for each major wind event in each AGN activity cycle, which is a fractal behavior. We then introduced a simple cellular automaton to investigate how the dynamical properties of an idealized disk-wind system change following the introduction of simple feedback rules. We find that without feedback, the system is overcritical. Conversely, when feedback is present, regardless of whether it is magnetic or radiation driven, the system can be driven toward a self-organized critical state. Conclusions. Our results corroborate the hypothesis that AGN feedback is a necessary key ingredient in disk-wind systems, and following this, in shaping the coevolution of galaxies and supermassive BHs.