Acoustic waves and g-mode turbulence as energy carriers in a viscous intracluster medium

TL;DR

This study uses 3D hydrodynamic simulations to compare how sound waves and turbulence dissipate energy in galaxy cluster cores, revealing their roles depend on feedback modes and highlighting the potential observational challenges in detecting sound waves.

Contribution

First 3D simulation demonstrating the relative contributions of sound waves and turbulence in energy dissipation under different feedback regimes in galaxy clusters.

Findings

Sound flux can carry up to 20% of injected power in intermediate feedback.

Turbulence contributes more in slow-piston regimes, less in intermediate regimes.

Sound waves may be difficult to detect based on the equation-of-state assumptions from X-ray data.

Abstract

Many recent works on the observed galaxy clusters in the X-rays highlight broadly two classes of exclusive energy carriers - sound waves and turbulence. In order to understand this dichotomy, we design an idealized three-dimensional hydrodynamic simulation of a cluster, to assess which of these carriers can dissipate energy in and around the core ($\gtrsim 100$ kpc) . Specifically, we explore how gentle (long-duration outbursts) and intermediate (shorter duration outbursts) feedback modes can function efficiently mediated by compressible (sound waves) and incompressible (g-modes/instabilities/turbulence) disturbances. Since g-modes are confined tightly to the central core, we attempt to maximise the flux of fast sound waves to distribute the feedback energy over a large distance. We find that the contribution to heat dissipation from sound and turbulence varies on the basis of the aforementioned feedback modes, namely: turbulence contributes relatively more than sound in the slow-piston regime and vice versa for the intermediate regime. For the first time in a 3D simulation, we show that up to $\lesssim 20\%$ (in some directions) of the injected power can be carried away by sound flux in the intermediate feedback but it reduces to $\lesssim 10 \%$ (in some directions) in the slow-piston regime. Lastly, we find that sound waves can be elusive if we deduce the equation-of-state (isobaric/isentropic) of the fluctuations from X-ray observations.