Subgrid Modeling of AGN-Driven Turbulence in Galaxy Clusters

TL;DR

This paper introduces a subgrid turbulence model in 3D simulations of AGN bubbles in galaxy clusters, revealing that turbulence and instabilities significantly influence bubble evolution and ICM properties.

Contribution

The study develops a novel subgrid turbulence model for AGN-driven bubbles, improving the understanding of their dynamics and impact on the intracluster medium.

Findings

Bubbles are transformed into mixed hot clouds as they rise.

Proper turbulence modeling preserves bubble coherence longer.

Impacts on entropy and metal distribution in galaxy clusters.

Abstract

Hot, underdense bubbles powered by active galactic nuclei (AGN) are likely to play a key role in halting catastrophic cooling in the centers of cool-core galaxy clusters. We present three-dimensional simulations that capture the evolution of such bubbles, using an adaptive-mesh hydrodynamic code, FLASH3, to which we have added a subgrid model of turbulence and mixing. While pure-hydro simulations indicate that AGN bubbles are disrupted into resolution-dependent pockets of underdense gas, proper modeling of subgrid turbulence indicates that this a poor approximation to a turbulent cascade that continues far beyond the resolution limit. Instead, Rayleigh-Taylor instabilities act to effectively mix the heated region with its surroundings, while at the same time preserving it as a coherent structure, consistent with observations. Thus bubbles are transformed into hot clouds of mixed material as they move outwards in the hydrostatic intracluster medium (ICM), much as large airbursts lead to a distinctive ``mushroom cloud'' structure as they rise in the hydrostatic atmosphere of Earth. Properly capturing the evolution of such clouds has important implications for many ICM properties. In particular, it significantly changes the impact of AGN on the distribution of entropy and metals in cool-core clusters such as Perseus.