Investigating the intracluster medium viscosity using the tails of GASP jellyfish galaxies

TL;DR

This study uses jellyfish galaxy tails to measure the intracluster medium's viscosity, finding it to be significantly lower than classical expectations, which suggests collisionless physics or suppressed particle mean free paths.

Contribution

It provides the first systematic analysis of ICM viscosity using galaxy tail velocity structures, constraining viscosity to a fraction of the Spitzer value.

Findings

ICM viscosity is 0.3-25% of the Spitzer value.

Turbulent motions extend below expected dissipation scales.

Results imply smaller particle mean free paths or collisionless effects.

Abstract

The microphysics of the intracluster medium (ICM) in galaxy clusters is still poorly understood. Observational evidence suggests that the effective viscosity is suppressed by plasma instabilities that reduce the mean free path of particles. Measuring the effective viscosity of the ICM is crucial to understanding the processes that govern its physics on small scales. The trails of ionized interstellar medium left behind by the so-called jellyfish galaxies can trace the turbulent motions of the surrounding ICM and constrain its local viscosity. We present the results of a systematic analysis of the velocity structure function (VSF) of the H$\alpha$ line for ten galaxies from the GASP sample. The VSFs show a sub-linear power law scaling below 10 kpc which may result from turbulent cascading and extends to 1 kpc, below the supposed ICM dissipation scales of tens of kpc expected in a fluid described by Coulomb collisions. Our result constrains the local ICM viscosity to be 0.3-25$\%$ of the expected Spitzer value. Our findings demonstrate that either the ICM particles have a smaller mean free path than expected in a regime defined by Coulomb collisions, or that we are probing effects due to collisionless physics in the ICM turbulence.