Publications associated with Theoretical Astrophysics & Plasma Physics
Suppressed effective viscosity in the bulk intergalactic plasma
Nature Astronomy Nature Research 3 (2019) 832–837-
<p>Transport properties, such as viscosity and thermal conduction, of the hot intergalactic plasma in clusters of galaxies are largely unknown. Whereas for laboratory plasmas these characteristics are derived from the gas density and temperature<sup>1</sup>, such recipes can be fundamentally different for the intergalactic plasma<sup>2</sup> owing to a low rate of particle collisions and a weak magnetic field<sup>3</sup>. In numerical simulations, these unknowns can often be avoided by modelling these plasmas as hydrodynamic fluids<sup>4,5,6</sup>, even though local, non-hydrodynamic features observed in clusters contradict this assumption<sup>7,8,9</sup>. Using deep Chandra observations of the Coma Cluster<sup>10,11</sup>, we probe gas fluctuations in intergalactic medium down to spatial scales where the transport processes should prominently manifest themselves—provided that hydrodynamic models<sup>12</sup> with pure Coulomb collision rates are indeed adequate. We do not find evidence of such transport processes, implying that the effective isotropic viscosity is orders of magnitude smaller than naively expected. This indicates either an enhanced collision rate in the plasma due to particle scattering off microfluctuations caused by plasma instabilities<sup>2,13,14</sup> or that the transport processes are anisotropic with respect to the local magnetic field<sup>15</sup>. This also means that numerical models with high Reynolds number appear more consistent with observations. Our results demonstrate that observations of turbulence in clusters<sup>16,17</sup> are giving rise to a branch of astrophysics that can sharpen theoretical views on galactic plasmas.</p>
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