Turbulent density and pressure fluctuations in the stratified intracluster medium

TL;DR

This study uses high-resolution simulations to explore how density and pressure fluctuations relate to turbulent motions in the stratified intracluster medium, developing new scaling relations that account for stratification effects.

Contribution

The paper introduces a novel scaling relation for density fluctuations in stratified turbulence, extending turbulence theory to the ICM context with comprehensive simulation data.

Findings

Density fluctuations depend on Mach number, stratification, and entropy-pressure scale height ratio.

Pressure fluctuations are independent of stratification and scale with Mach number.

New analytical relations connect turbulence parameters to observable fluctuations in galaxy clusters.

Abstract

Turbulent gas motions are observed in the intracluster medium (ICM). The ICM is density-stratified, with the gas density being highest at the centre of the cluster and decreasing radially outwards. As a result of this, Kolmogorov (homogeneous, isotropic) turbulence theory does not apply to the ICM. The gas motions are instead explained by anisotropic stratified turbulence, with the stratification quantified by the perpendicular Froude number ($\mathrm{Fr}_\perp$). These turbulent motions are associated with density and pressure fluctuations, which manifest as perturbations in X-ray surface brightness maps of the ICM and as thermal Sunyaev-Zeldovich effect (SZ) fluctuations, respectively. In order to advance our understanding of the relations between these fluctuations and the turbulent gas velocities, we have conducted 100 high-resolution hydrodynamic simulations of stratified turbulence ($256^2\times 384$ -- $1024^2\times1536$ resolution elements), in which we scan the parameter space of subsonic rms Mach number ($\mathcal{M}$), $\mathrm{Fr}_\perp$, and the ratio of entropy and pressure scale heights ($R_{PS}=H_P/H_S$), relevant to the ICM. We develop a new scaling relation between the standard deviation of logarithmic density fluctuations ($\sigma_s$, where $s=\ln(\rho/\left<\rho\right>)$), $\mathcal{M}$, and $\mathrm{Fr}_{\perp}$, valid till $\mathrm{Fr}_\perp\ll1$:~$\sigma_s^2=\ln\left(1+b^2\mathcal{M}^4+0.10/(\mathrm{Fr}_\perp+0.25/\sqrt{\mathrm{Fr}_\perp})^2\mathcal{M}^2R_{PS}\right)$, where $b\sim1/3$ for solenoidal turbulence driving studied here. We further find that logarithmic pressure fluctuations $\sigma_{(\ln{P}/\left<P\right>)}$ are independent of stratification and scale according to the relation $\sigma_{(\ln{\bar{P}})}^2=\ln\left(1+b^2\gamma^2\mathcal{M}^4\right)$, where $\bar{P}=P/\left<P\right>$ and $\gamma$ is the adiabatic index of the gas.