Iron Opacity Bump Changes the Stability and Structure of Accretion Disks in Active Galactic Nuclei

TL;DR

This study demonstrates that the iron opacity bump significantly influences the thermal stability and vertical structure of AGN accretion disks, potentially explaining observed discrepancies and linking metallicity to AGN properties.

Contribution

First 3D radiation MHD simulations showing the stabilizing effect of iron opacity bump on radiation pressure dominated disks.

Findings

Iron opacity bump prevents thermal runaway in simulations.

Disks with iron opacity maintain structure over multiple thermal times.

Enhanced vertical energy transport is observed with iron opacity.

Abstract

Accretion disks around supermassive black holes have regions where the Rosseland mean opacity can be much larger than the electron scattering opacity primarily due to the large number of bound-bound transitions in iron. We study the effects of this iron opacity "bump" on the thermal stability and vertical structure of radiation pressure dominated accretion disks, utilizing three dimensional radiation magneto-hydrodynamic simulations in the local shearing box approximation. The simulations self-consistently calculate the heating due to MHD turbulence caused by magneto-rotational instability and radiative cooling by using the radiative transfer module based on a variable Eddington tensor in Athena. For a $5\times 10^8$ solar mass black hole with $\sim 3\%$ of the Eddington luminosity, a model including the iron opacity bump maintains its structure for more than $10$ thermal times without showing significant signs of thermal runaway. In contrast, if only electron scattering and free-free opacity are included as in the standard thin disk model, the disk collapses on the thermal time scale. The difference is caused by a combination of 1) an anti-correlation between the total optical depth and the temperature, and 2) enhanced vertical advective energy transport. These results suggest that the iron opacity bump may have a strong impact on the stability and structure of AGN accretion disks, and may contribute to a dependence of AGN properties on metallicity. Since this opacity is relevant primarily in UV emitting regions of the flow, it may help to explain discrepancies between observation and theory that are unique to AGNs.