Turbulence in the intracluster medium: simulations, observables & thermodynamics

TL;DR

This study uses simulations to explore turbulence and thermodynamics in the intracluster medium, revealing how turbulence affects observable fluctuations and constraining turbulent heating's role in cool core clusters.

Contribution

It provides new insights into turbulence scaling laws, the impact of thermal balance on gas phases, and constraints on turbulent heating based on simulations and observations.

Findings

Density and surface brightness fluctuations scale with Mach number squared in pure turbulence.

Turbulent velocities exceed observational limits unless turbulent heating is minimal.

Multiphase gas formation depends on driving scale and timescales.

Abstract

We conduct two kinds of homogeneous isotropic turbulence simulations relevant for the intracluster medium (ICM): (i) pure turbulence runs without radiative cooling; (ii) turbulent heating$+$radiative cooling runs with global thermal balance. For pure turbulence runs in the subsonic regime, the rms density and surface brightness (SB) fluctuations vary as the square of the rms Mach number ($\mathcal{M}_{\text{rms}}$). However, with thermal balance, the density and SB fluctuations $(\delta SB/SB)$ are much larger. These scalings have implications for translating SB fluctuations into a turbulent velocity, particularly for cool cores. For thermal balance runs with large (cluster core) scale driving, both the hot and cold phases of the gas are supersonic. For small scale (one order of magnitude smaller than the cluster core) driving, multiphase gas forms on a much longer timescale but $\mathcal{M}_{\text{rms}}$ is smaller. Both small and large scale driving runs have velocities larger than the Hitomi results from the Perseus cluster. Thus turbulent heating as the dominant heating source in cool cluster cores is ruled out if multiphase gas is assumed to condense out from the ICM. Next we perform thermal balance runs in which we partition the input energy into thermal and turbulent parts and tune their relative magnitudes. The contribution of turbulent heating has to be $\lesssim 10\%$ in order for turbulence velocities to match Hitomi observations. If the dominant source of multiphase gas is not cooling from the ICM (but say uplift from the central galaxy), the importance of turbulent heating cannot be excluded.