Suppressed effective viscosity in the bulk intergalactic plasma

TL;DR

This study uses deep X-ray observations of the Coma Cluster to show that the effective viscosity in intergalactic plasma is much lower than expected, suggesting microfluctuations or magnetic effects influence plasma transport.

Contribution

The paper provides observational evidence that the effective viscosity in intergalactic plasma is suppressed, challenging standard hydrodynamic models based on Coulomb collisions.

Findings

Effective viscosity is orders of magnitude smaller than expected.

Microfluctuations or magnetic fields likely influence plasma transport.

High Reynolds number models align better with observations.

Abstract

Transport properties, such as viscosity and thermal conduction, of the hot intergalactic plasma in clusters of galaxies, are largely unknown. While for laboratory plasmas these characteristics are derived from the gas density and temperature, such recipes can be fundamentally different for the intergalactic plasma due to a low rate of particle collisions and a weak magnetic field. In numerical simulations, one often cuts through these unknowns by modeling these plasmas as hydrodynamic fluids, even though local, non-hydrodynamic features observed in clusters contradict this assumption. Using deep Chandra observations of the Coma Cluster, we probe gas fluctuations in intergalactic medium down to spatial scales where the transport processes should prominently manifest themselves - at least if hydrodynamic models with pure Coulomb collision rates were indeed adequate. We find that they do not, implying that the effective isotropic viscosity is orders of magnitude smaller than naively expected. This indicates an enhanced collision rate in the plasma due to particle scattering off microfluctuations caused by plasma instabilities, or that the transport processes are anisotropic with respect to local magnetic field. For that reason, numerical models with high Reynolds number appear more consistent with observations. Our results also demonstrate that observations of turbulence in clusters are becoming a branch of astrophysics that can sharpen theoretical views on such plasmas.