Cooling+Heating Flows in Galaxy clusters: Turbulent heating, spectral modelling, and cooling efficiency

TL;DR

This paper proposes a model where MHD turbulent viscous heating, characterized by a viscosity parameter, balances cooling in galaxy clusters, improving spectral fits and aligning with turbulence measurements.

Contribution

It introduces a cooling+heating flow model with a specific viscosity parameter that better explains X-ray spectra and cooling efficiency in galaxy clusters.

Findings

A viscosity parameter of ~0.05 improves spectral fits.

The model predicts a cooling efficiency of about 0.33.

Inferred viscosity aligns with turbulence measurements.

Abstract

The discrepancy between expected and observed cooling rates of X-ray emitting gas has led to the {\it cooling flow problem} at the cores of clusters of galaxies. A variety of models have been proposed to model the observed X-ray spectra and resolve the cooling flow problem, which involves heating the cold gas through different mechanisms. As a result, realistic models of X-ray spectra of galaxy clusters need to involve both heating {\it and} cooling mechanisms. In this paper, we argue that the heating time-scale is set by the magnetohydrodynamic (MHD) turbulent viscous heating for the Intracluster plasma, parametrised by the Shakura-Sunyaev viscosity parameter, $\alpha$. Using a cooling+heating flow model, we show that a value of $\alpha\simeq 0.05$ (with 10\% scatter) provides improved fits to the X-ray spectra of cooling flow, while at the same time, predicting reasonable cooling efficiency, $\epsilon_{cool} = 0.33^{+0.63}_{-0.15}$. Our inferred values for $\alpha$ based on X-ray spectra are also in line with direct measurements of turbulent pressure in simulations and observations of galaxy clusters. This simple picture unifies astrophysical accretion, as a balance of MHD turbulent heating and cooling, across more than 16 orders of magnitudes in scale, from neutron stars to galaxy clusters.