Rayleigh-Taylor instability of ionization front around black holes

TL;DR

This study uses radiation-hydrodynamic simulations to demonstrate that ionizing radiation from black holes suppresses Rayleigh-Taylor instabilities at the ionization front, with stability depending on the relation between the Strömgren and Bondi radii.

Contribution

It provides the first detailed simulation-based analysis of RTI suppression by black hole radiation, highlighting conditions for stability and the impact of accretion bursts.

Findings

RTI is suppressed for non-accelerating ionization fronts.

Stability requires the Strömgren sphere radius to exceed the Bondi radius.

Accretion bursts cause transient RTI growth during rapid ionization front expansion.

Abstract

We examine the role of ionizing radiation emitted from black holes (BHs) in suppressing the growth of the Rayleigh-Taylor instability (RTI) across the ionization front (I-front) that forms when the gas fueling the BH is neutral. We use radiation-hydrodynamic simulations to show that the RTI is suppressed for non-accelerating fronts on all scales resolved in our simulations. A necessary condition for the stability of the I-front is that the radius of the Str\"omgren sphere is larger than the Bondi radius. When this condition is violated the I-front collapses producing an accretion luminosity burst. Transient growth of the RTI occurs only during the accretion burst when the effective acceleration in the frame of reference of the I-front increases significantly due to the rapid expansion of the Str\"omgren sphere.